diff --git a/src/main/java/com/google/devtools/build/lib/rules/python/PyRuntime.java b/src/main/java/com/google/devtools/build/lib/rules/python/PyRuntime.java
index c5a904e..4459f39 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/python/PyRuntime.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/python/PyRuntime.java
@@ -14,6 +14,7 @@
 
 package com.google.devtools.build.lib.rules.python;
 
+import com.google.common.base.Preconditions;
 import com.google.devtools.build.lib.actions.Artifact;
 import com.google.devtools.build.lib.actions.MutableActionGraph.ActionConflictException;
 import com.google.devtools.build.lib.analysis.ConfiguredTarget;
@@ -37,6 +38,9 @@
     Artifact interpreter = ruleContext.getPrerequisiteArtifact("interpreter", Mode.TARGET);
     PathFragment interpreterPath =
         PathFragment.create(ruleContext.attributes().get("interpreter_path", Type.STRING));
+    PythonVersion pythonVersion =
+        PythonVersion.parseTargetOrSentinelValue(
+            ruleContext.attributes().get("python_version", Type.STRING));
 
     // Determine whether we're pointing to an in-build target (hermetic) or absolute system path
     // (non-hermetic).
@@ -53,14 +57,21 @@
       ruleContext.attributeError("interpreter_path", "must be an absolute path.");
     }
 
+    if (pythonVersion == PythonVersion._INTERNAL_SENTINEL) {
+      // Use the same default as py_binary/py_test would use for their python_version attribute.
+      // (Of course, in our case there's no configuration transition involved.)
+      pythonVersion = ruleContext.getFragment(PythonConfiguration.class).getDefaultPythonVersion();
+    }
+    Preconditions.checkState(pythonVersion.isTargetValue());
+
     if (ruleContext.hasErrors()) {
       return null;
     }
 
     PyRuntimeInfo provider =
         hermetic
-            ? PyRuntimeInfo.createForInBuildRuntime(interpreter, files)
-            : PyRuntimeInfo.createForPlatformRuntime(interpreterPath);
+            ? PyRuntimeInfo.createForInBuildRuntime(interpreter, files, pythonVersion)
+            : PyRuntimeInfo.createForPlatformRuntime(interpreterPath, pythonVersion);
 
     return new RuleConfiguredTargetBuilder(ruleContext)
         .setFilesToBuild(files)
diff --git a/src/main/java/com/google/devtools/build/lib/rules/python/PyRuntimeInfo.java b/src/main/java/com/google/devtools/build/lib/rules/python/PyRuntimeInfo.java
index 4dab2b3..bfa60dc 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/python/PyRuntimeInfo.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/python/PyRuntimeInfo.java
@@ -52,15 +52,19 @@
   @Nullable private final PathFragment interpreterPath;
   @Nullable private final Artifact interpreter;
   @Nullable private final SkylarkNestedSet files;
+  /** Invariant: either PY2 or PY3. */
+  private final PythonVersion pythonVersion;
 
   private PyRuntimeInfo(
       @Nullable Location location,
       @Nullable PathFragment interpreterPath,
       @Nullable Artifact interpreter,
-      @Nullable SkylarkNestedSet files) {
+      @Nullable SkylarkNestedSet files,
+      PythonVersion pythonVersion) {
     super(PROVIDER, location);
     Preconditions.checkArgument((interpreterPath == null) != (interpreter == null));
     Preconditions.checkArgument((interpreter == null) == (files == null));
+    Preconditions.checkArgument(pythonVersion.isTargetValue());
     if (files != null) {
       // Work around #7266 by special-casing the empty set in the type check.
       Preconditions.checkArgument(
@@ -69,22 +73,25 @@
     this.interpreterPath = interpreterPath;
     this.interpreter = interpreter;
     this.files = files;
+    this.pythonVersion = pythonVersion;
   }
 
   /** Constructs an instance from native rule logic (built-in location) for an in-build runtime. */
   public static PyRuntimeInfo createForInBuildRuntime(
-      Artifact interpreter, NestedSet<Artifact> files) {
+      Artifact interpreter, NestedSet<Artifact> files, PythonVersion pythonVersion) {
     return new PyRuntimeInfo(
         /*location=*/ null,
         /*interpreterPath=*/ null,
         interpreter,
-        SkylarkNestedSet.of(Artifact.class, files));
+        SkylarkNestedSet.of(Artifact.class, files),
+        pythonVersion);
   }
 
   /** Constructs an instance from native rule logic (built-in location) for a platform runtime. */
-  public static PyRuntimeInfo createForPlatformRuntime(PathFragment interpreterPath) {
+  public static PyRuntimeInfo createForPlatformRuntime(
+      PathFragment interpreterPath, PythonVersion pythonVersion) {
     return new PyRuntimeInfo(
-        /*location=*/ null, interpreterPath, /*interpreter=*/ null, /*files=*/ null);
+        /*location=*/ null, interpreterPath, /*interpreter=*/ null, /*files=*/ null, pythonVersion);
   }
 
   @Override
@@ -148,6 +155,15 @@
     return files;
   }
 
+  public PythonVersion getPythonVersion() {
+    return pythonVersion;
+  }
+
+  @Override
+  public String getPythonVersionForStarlark() {
+    return pythonVersion.name();
+  }
+
   /** The class of the {@code PyRuntimeInfo} provider type. */
   public static class PyRuntimeInfoProvider extends BuiltinProvider<PyRuntimeInfo>
       implements PyRuntimeInfoApi.PyRuntimeInfoProviderApi {
@@ -158,7 +174,11 @@
 
     @Override
     public PyRuntimeInfo constructor(
-        Object interpreterPathUncast, Object interpreterUncast, Object filesUncast, Location loc)
+        Object interpreterPathUncast,
+        Object interpreterUncast,
+        Object filesUncast,
+        String pythonVersion,
+        Location loc)
         throws EvalException {
       String interpreterPath =
           interpreterPathUncast == NONE ? null : (String) interpreterPathUncast;
@@ -175,15 +195,27 @@
         throw new EvalException(loc, "cannot specify 'files' if 'interpreter_path' is given");
       }
 
+      PythonVersion parsedPythonVersion;
+      try {
+        parsedPythonVersion = PythonVersion.parseTargetValue(pythonVersion);
+      } catch (IllegalArgumentException ex) {
+        throw new EvalException(loc, "illegal value for 'python_version'", ex);
+      }
+
       if (isInBuildRuntime) {
         if (files == null) {
           files =
               SkylarkNestedSet.of(Artifact.class, NestedSetBuilder.emptySet(Order.STABLE_ORDER));
         }
-        return new PyRuntimeInfo(loc, /*interpreterPath=*/ null, interpreter, files);
+        return new PyRuntimeInfo(
+            loc, /*interpreterPath=*/ null, interpreter, files, parsedPythonVersion);
       } else {
         return new PyRuntimeInfo(
-            loc, PathFragment.create(interpreterPath), /*interpreter=*/ null, /*files=*/ null);
+            loc,
+            PathFragment.create(interpreterPath),
+            /*interpreter=*/ null,
+            /*files=*/ null,
+            parsedPythonVersion);
       }
     }
   }
diff --git a/src/main/java/com/google/devtools/build/lib/rules/python/PyRuntimeRule.java b/src/main/java/com/google/devtools/build/lib/rules/python/PyRuntimeRule.java
index 5126ce9..1a92a32 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/python/PyRuntimeRule.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/python/PyRuntimeRule.java
@@ -33,6 +33,9 @@
   public RuleClass build(RuleClass.Builder builder, RuleDefinitionEnvironment env) {
     return builder
 
+        // For --incompatible_py3_is_default.
+        .requiresConfigurationFragments(PythonConfiguration.class)
+
         /* <!-- #BLAZE_RULE(py_runtime).ATTRIBUTE(files) -->
         For an in-build runtime, this is the set of files comprising this runtime. These files will
         be added to the runfiles of Python binaries that use this runtime. For a platform runtime
@@ -51,6 +54,18 @@
         platform. For an in-build runtime this attribute must not be set.
         <!-- #END_BLAZE_RULE.ATTRIBUTE --> */
         .add(attr("interpreter_path", STRING))
+
+        /* <!-- #BLAZE_RULE(py_runtime).ATTRIBUTE(python_version) -->
+        Whether this runtime is for Python major version 2 or 3. Valid values are <code>"PY2"</code>
+        and <code>"PY3"</code>.
+
+        <p>The default value is controlled by the <code>--incompatible_py3_is_default</code> flag.
+        However, in the future this attribute will be mandatory and have no default value.
+        <!-- #END_BLAZE_RULE.ATTRIBUTE --> */
+        .add(
+            attr("python_version", STRING)
+                .value(PythonVersion._INTERNAL_SENTINEL.toString())
+                .allowedValues(PyRuleClasses.TARGET_PYTHON_ATTR_VALUE_SET))
         .add(attr("output_licenses", LICENSE))
         .build();
   }
diff --git a/src/main/java/com/google/devtools/build/lib/rules/python/PythonConfiguration.java b/src/main/java/com/google/devtools/build/lib/rules/python/PythonConfiguration.java
index c8d68d6a..7dcc069 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/python/PythonConfiguration.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/python/PythonConfiguration.java
@@ -39,6 +39,7 @@
 public class PythonConfiguration extends BuildConfiguration.Fragment {
 
   private final PythonVersion version;
+  private final PythonVersion defaultVersion;
   private final TriState buildPythonZip;
   private final boolean buildTransitiveRunfilesTrees;
 
@@ -51,12 +52,14 @@
 
   PythonConfiguration(
       PythonVersion version,
+      PythonVersion defaultVersion,
       TriState buildPythonZip,
       boolean buildTransitiveRunfilesTrees,
       boolean oldPyVersionApiAllowed,
       boolean useNewPyVersionSemantics,
       boolean disallowLegacyPyProvider) {
     this.version = version;
+    this.defaultVersion = defaultVersion;
     this.buildPythonZip = buildPythonZip;
     this.buildTransitiveRunfilesTrees = buildTransitiveRunfilesTrees;
     this.oldPyVersionApiAllowed = oldPyVersionApiAllowed;
@@ -75,6 +78,21 @@
     return version;
   }
 
+  /**
+   * Returns the default Python version to use on targets that omit their {@code python_version}
+   * attribute.
+   *
+   * <p>Specified using {@code --incompatible_py3_is_default}. Long-term, the default will simply be
+   * hardcoded as {@code PY3}.
+   *
+   * <p>This information is stored on the configuration for the benefit of callers in rule analysis.
+   * However, transitions have access to the option fragment instead of the configuration fragment,
+   * and should rely on {@link PythonOptions#getDefaultPythonVersion} instead.
+   */
+  public PythonVersion getDefaultPythonVersion() {
+    return defaultVersion;
+  }
+
   @Override
   public String getOutputDirectoryName() {
     // TODO(brandjon): Implement alternative semantics for controlling which python version(s) get
diff --git a/src/main/java/com/google/devtools/build/lib/rules/python/PythonConfigurationLoader.java b/src/main/java/com/google/devtools/build/lib/rules/python/PythonConfigurationLoader.java
index 08095e7..c5c2fba 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/python/PythonConfigurationLoader.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/python/PythonConfigurationLoader.java
@@ -36,6 +36,7 @@
     PythonVersion pythonVersion = pythonOptions.getPythonVersion();
     return new PythonConfiguration(
         pythonVersion,
+        pythonOptions.getDefaultPythonVersion(),
         pythonOptions.buildPythonZip,
         pythonOptions.buildTransitiveRunfilesTrees,
         /*oldPyVersionApiAllowed=*/ !pythonOptions.incompatibleRemoveOldPythonVersionApi,
diff --git a/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/python/PyRuntimeInfoApi.java b/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/python/PyRuntimeInfoApi.java
index ebd79d1..b1ccaad 100644
--- a/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/python/PyRuntimeInfoApi.java
+++ b/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/python/PyRuntimeInfoApi.java
@@ -78,6 +78,14 @@
   @Nullable
   SkylarkNestedSet getFilesForStarlark();
 
+  @SkylarkCallable(
+      name = "python_version",
+      structField = true,
+      doc =
+          "Indicates whether this runtime uses Python major version 2 or 3. Valid values are "
+              + "(only) <code>\"PY2\"</code> and <code>\"PY3\"</code>.")
+  String getPythonVersionForStarlark();
+
   /** Provider type for {@link PyRuntimeInfoApi} objects. */
   @SkylarkModule(name = "Provider", documented = false, doc = "")
   interface PyRuntimeInfoProviderApi extends ProviderApi {
@@ -119,12 +127,22 @@
                       + "for this argument if you pass in <code>interpreter_path</code>. If "
                       + "<code>interpreter</code> is given and this argument is <code>None</code>, "
                       + "<code>files</code> becomes an empty <code>depset</code> instead."),
+          @Param(
+              name = "python_version",
+              type = String.class,
+              positional = false,
+              named = true,
+              doc = "The value for the new object's <code>python_version</code> field."),
         },
         selfCall = true,
         useLocation = true)
     @SkylarkConstructor(objectType = PyRuntimeInfoApi.class, receiverNameForDoc = "PyRuntimeInfo")
     PyRuntimeInfoApi<?> constructor(
-        Object interpreterPathUncast, Object interpreterUncast, Object filesUncast, Location loc)
+        Object interpreterPathUncast,
+        Object interpreterUncast,
+        Object filesUncast,
+        String pythonVersion,
+        Location loc)
         throws EvalException;
   }
 }
diff --git a/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/python/FakePyRuntimeInfo.java b/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/python/FakePyRuntimeInfo.java
index 8ab56ab..7ba31e7 100644
--- a/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/python/FakePyRuntimeInfo.java
+++ b/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/python/FakePyRuntimeInfo.java
@@ -40,6 +40,11 @@
   }
 
   @Override
+  public String getPythonVersionForStarlark() {
+    return "";
+  }
+
+  @Override
   public void repr(SkylarkPrinter printer) {}
 
   /** Fake implementation of {@link PyRuntimeInfoProviderApi}. */
@@ -47,7 +52,11 @@
 
     @Override
     public PyRuntimeInfoApi<?> constructor(
-        Object interpreterPathUncast, Object interpreterUncast, Object filesUncast, Location loc)
+        Object interpreterPathUncast,
+        Object interpreterUncast,
+        Object filesUncast,
+        String pythonVersion,
+        Location loc)
         throws EvalException {
       return new FakePyRuntimeInfo();
     }
diff --git a/src/test/java/com/google/devtools/build/lib/rules/python/BUILD b/src/test/java/com/google/devtools/build/lib/rules/python/BUILD
index bd56e2e..e5b1b9e 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/python/BUILD
+++ b/src/test/java/com/google/devtools/build/lib/rules/python/BUILD
@@ -58,6 +58,7 @@
     name = "PyRuntimeTest",
     srcs = ["PyRuntimeTest.java"],
     deps = [
+        ":PythonTestUtils",
         "//src/main/java/com/google/devtools/build/lib:bazel-rules",
         "//src/main/java/com/google/devtools/build/lib:python-rules",
         "//src/test/java/com/google/devtools/build/lib:actions_testutil",
diff --git a/src/test/java/com/google/devtools/build/lib/rules/python/PyRuntimeInfoTest.java b/src/test/java/com/google/devtools/build/lib/rules/python/PyRuntimeInfoTest.java
index 7da0c00..495cd6e 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/python/PyRuntimeInfoTest.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/python/PyRuntimeInfoTest.java
@@ -54,7 +54,8 @@
   @Test
   public void factoryMethod_InBuildRuntime() {
     NestedSet<Artifact> files = NestedSetBuilder.create(Order.STABLE_ORDER, dummyFile);
-    PyRuntimeInfo inBuildRuntime = PyRuntimeInfo.createForInBuildRuntime(dummyInterpreter, files);
+    PyRuntimeInfo inBuildRuntime =
+        PyRuntimeInfo.createForInBuildRuntime(dummyInterpreter, files, PythonVersion.PY2);
 
     assertThat(inBuildRuntime.getCreationLoc()).isEqualTo(Location.BUILTIN);
     assertThat(inBuildRuntime.getInterpreterPath()).isNull();
@@ -62,12 +63,14 @@
     assertThat(inBuildRuntime.getInterpreter()).isEqualTo(dummyInterpreter);
     assertThat(inBuildRuntime.getFiles()).isEqualTo(files);
     assertThat(inBuildRuntime.getFilesForStarlark().getSet(Artifact.class)).isEqualTo(files);
+    assertThat(inBuildRuntime.getPythonVersion()).isEqualTo(PythonVersion.PY2);
+    assertThat(inBuildRuntime.getPythonVersionForStarlark()).isEqualTo("PY2");
   }
 
   @Test
   public void factoryMethod_PlatformRuntime() {
     PathFragment path = PathFragment.create("/system/interpreter");
-    PyRuntimeInfo platformRuntime = PyRuntimeInfo.createForPlatformRuntime(path);
+    PyRuntimeInfo platformRuntime = PyRuntimeInfo.createForPlatformRuntime(path, PythonVersion.PY2);
 
     assertThat(platformRuntime.getCreationLoc()).isEqualTo(Location.BUILTIN);
     assertThat(platformRuntime.getInterpreterPath()).isEqualTo(path);
@@ -75,6 +78,8 @@
     assertThat(platformRuntime.getInterpreter()).isNull();
     assertThat(platformRuntime.getFiles()).isNull();
     assertThat(platformRuntime.getFilesForStarlark()).isNull();
+    assertThat(platformRuntime.getPythonVersion()).isEqualTo(PythonVersion.PY2);
+    assertThat(platformRuntime.getPythonVersionForStarlark()).isEqualTo("PY2");
   }
 
   @Test
@@ -83,12 +88,14 @@
         "info = PyRuntimeInfo(",
         "    interpreter = dummy_interpreter,",
         "    files = depset([dummy_file]),",
+        "    python_version = 'PY2',",
         ")");
     PyRuntimeInfo info = (PyRuntimeInfo) lookup("info");
     assertThat(info.getCreationLoc().getStartOffset()).isEqualTo(7);
     assertThat(info.getInterpreterPath()).isNull();
     assertThat(info.getInterpreter()).isEqualTo(dummyInterpreter);
     assertHasOrderAndContainsExactly(info.getFiles(), Order.STABLE_ORDER, dummyFile);
+    assertThat(info.getPythonVersion()).isEqualTo(PythonVersion.PY2);
   }
 
   @Test
@@ -96,12 +103,14 @@
     eval(
         "info = PyRuntimeInfo(", //
         "    interpreter_path = '/system/interpreter',",
+        "    python_version = 'PY2',",
         ")");
     PyRuntimeInfo info = (PyRuntimeInfo) lookup("info");
     assertThat(info.getCreationLoc().getStartOffset()).isEqualTo(7);
     assertThat(info.getInterpreterPath()).isEqualTo(PathFragment.create("/system/interpreter"));
     assertThat(info.getInterpreter()).isNull();
     assertThat(info.getFiles()).isNull();
+    assertThat(info.getPythonVersion()).isEqualTo(PythonVersion.PY2);
   }
 
   @Test
@@ -109,6 +118,7 @@
     eval(
         "info = PyRuntimeInfo(", //
         "    interpreter = dummy_interpreter,",
+        "    python_version = 'PY2',",
         ")");
     PyRuntimeInfo info = (PyRuntimeInfo) lookup("info");
     assertHasOrderAndContainsExactly(info.getFiles(), Order.STABLE_ORDER);
@@ -118,12 +128,15 @@
   public void starlarkConstructorErrors_InBuildXorPlatform() throws Exception {
     checkEvalErrorContains(
         "exactly one of the 'interpreter' or 'interpreter_path' arguments must be specified",
-        "PyRuntimeInfo()");
+        "PyRuntimeInfo(",
+        "    python_version = 'PY2',",
+        ")");
     checkEvalErrorContains(
         "exactly one of the 'interpreter' or 'interpreter_path' arguments must be specified",
         "PyRuntimeInfo(",
         "    interpreter_path = '/system/interpreter',",
         "    interpreter = dummy_interpreter,",
+        "    python_version = 'PY2',",
         ")");
   }
 
@@ -134,18 +147,37 @@
         "PyRuntimeInfo(",
         "    interpreter = dummy_interpreter,",
         "    files = 'abc',",
+        "    python_version = 'PY2',",
         ")");
     checkEvalErrorContains(
         "expected value of type 'depset of Files or NoneType' for parameter 'files'",
         "PyRuntimeInfo(",
         "    interpreter = dummy_interpreter,",
         "    files = depset(['abc']),",
+        "    python_version = 'PY2',",
         ")");
     checkEvalErrorContains(
         "cannot specify 'files' if 'interpreter_path' is given",
         "PyRuntimeInfo(",
         "    interpreter_path = '/system/interpreter',",
         "    files = depset([dummy_file]),",
+        "    python_version = 'PY2',",
+        ")");
+  }
+
+  @Test
+  public void starlarkConstructorErrors_PythonVersion() throws Exception {
+    checkEvalErrorContains(
+        "parameter 'python_version' has no default value",
+        "PyRuntimeInfo(",
+        "    interpreter_path = '/system/interpreter',",
+        ")");
+    checkEvalErrorContains(
+        "illegal value for 'python_version': 'not a Python version' is not a valid Python major "
+            + "version. Expected 'PY2' or 'PY3'.",
+        "PyRuntimeInfo(",
+        "    interpreter_path = '/system/interpreter',",
+        "    python_version = 'not a Python version',",
         ")");
   }
 }
diff --git a/src/test/java/com/google/devtools/build/lib/rules/python/PyRuntimeTest.java b/src/test/java/com/google/devtools/build/lib/rules/python/PyRuntimeTest.java
index 3c528f5..89a2293 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/python/PyRuntimeTest.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/python/PyRuntimeTest.java
@@ -15,6 +15,7 @@
 package com.google.devtools.build.lib.rules.python;
 
 import static com.google.common.truth.Truth.assertThat;
+import static com.google.devtools.build.lib.rules.python.PythonTestUtils.ensureDefaultIsPY2;
 
 import com.google.devtools.build.lib.actions.util.ActionsTestUtil;
 import com.google.devtools.build.lib.analysis.util.BuildViewTestCase;
@@ -40,6 +41,7 @@
         "    name = 'myruntime',",
         "    files = [':myfile'],",
         "    interpreter = ':myinterpreter',",
+        "    python_version = 'PY2',",
         ")");
     PyRuntimeInfo info = getConfiguredTarget("//pkg:myruntime").get(PyRuntimeInfo.PROVIDER);
 
@@ -47,6 +49,7 @@
     assertThat(info.getInterpreterPath()).isNull();
     assertThat(info.getInterpreter().getExecPathString()).isEqualTo("pkg/myinterpreter");
     assertThat(ActionsTestUtil.baseArtifactNames(info.getFiles())).containsExactly("myfile");
+    assertThat(info.getPythonVersion()).isEqualTo(PythonVersion.PY2);
   }
 
   @Test
@@ -56,6 +59,7 @@
         "py_runtime(",
         "    name = 'myruntime',",
         "    interpreter_path = '/system/interpreter',",
+        "    python_version = 'PY2',",
         ")");
     PyRuntimeInfo info = getConfiguredTarget("//pkg:myruntime").get(PyRuntimeInfo.PROVIDER);
 
@@ -63,6 +67,30 @@
     assertThat(info.getInterpreterPath().getPathString()).isEqualTo("/system/interpreter");
     assertThat(info.getInterpreter()).isNull();
     assertThat(info.getFiles()).isNull();
+    assertThat(info.getPythonVersion()).isEqualTo(PythonVersion.PY2);
+  }
+
+  @Test
+  public void pythonVersionDefault() throws Exception {
+    ensureDefaultIsPY2();
+    scratch.file(
+        "pkg/BUILD",
+        "py_runtime(",
+        "    name = 'myruntime_default',",
+        "    interpreter_path = '/system/interpreter',",
+        ")",
+        "py_runtime(",
+        "    name = 'myruntime_explicit',",
+        "    interpreter_path = '/system/interpreter',",
+        "    python_version = 'PY3',",
+        ")");
+    PyRuntimeInfo infoDefault =
+        getConfiguredTarget("//pkg:myruntime_default").get(PyRuntimeInfo.PROVIDER);
+    PyRuntimeInfo infoExplicit =
+        getConfiguredTarget("//pkg:myruntime_explicit").get(PyRuntimeInfo.PROVIDER);
+
+    assertThat(infoDefault.getPythonVersion()).isEqualTo(PythonVersion.PY2);
+    assertThat(infoExplicit.getPythonVersion()).isEqualTo(PythonVersion.PY3);
   }
 
   @Test
@@ -74,6 +102,7 @@
         "    name = 'myruntime',",
         "    interpreter = ':myinterpreter',",
         "    interpreter_path = '/system/interpreter',",
+        "    python_version = 'PY2',",
         ")");
     getConfiguredTarget("//pkg:myruntime");
 
@@ -88,6 +117,7 @@
         "pkg/BUILD", //
         "py_runtime(",
         "    name = 'myruntime',",
+        "    python_version = 'PY2',",
         ")");
     getConfiguredTarget("//pkg:myruntime");
 
@@ -103,6 +133,7 @@
         "py_runtime(",
         "    name = 'myruntime',",
         "    interpreter_path = 'some/relative/path',",
+        "    python_version = 'PY2',",
         ")");
     getConfiguredTarget("//pkg:myruntime");
 
@@ -118,9 +149,25 @@
         "    name = 'myruntime',",
         "    files = [':myfile'],",
         "    interpreter_path = '/system/interpreter',",
+        "    python_version = 'PY2',",
         ")");
     getConfiguredTarget("//pkg:myruntime");
 
     assertContainsEvent("if 'interpreter_path' is given then 'files' must be empty");
   }
+
+  @Test
+  public void badPythonVersionAttribute() throws Exception {
+    reporter.removeHandler(failFastHandler);
+    scratch.file(
+        "pkg/BUILD",
+        "py_runtime(",
+        "    name = 'myruntime',",
+        "    interpreter_path = '/system/interpreter',",
+        "    python_version = 'not a Python version',",
+        ")");
+    getConfiguredTarget("//pkg:myruntime");
+
+    assertContainsEvent("invalid value in 'python_version' attribute");
+  }
 }
diff --git a/src/test/java/com/google/devtools/build/lib/rules/python/PythonStarlarkApiTest.java b/src/test/java/com/google/devtools/build/lib/rules/python/PythonStarlarkApiTest.java
index c9cf589..89f7da4 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/python/PythonStarlarkApiTest.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/python/PythonStarlarkApiTest.java
@@ -140,7 +140,8 @@
         "    info = ctx.attr.runtime[PyRuntimeInfo]",
         "    return [PyRuntimeInfo(",
         "        interpreter = ctx.file.interpreter,",
-        "        files = depset(direct = ctx.files.files, transitive=[info.files]))]",
+        "        files = depset(direct = ctx.files.files, transitive=[info.files]),",
+        "        python_version = info.python_version)]",
         "",
         "userruntime = rule(",
         "    implementation = _userruntime_impl,",
@@ -157,6 +158,7 @@
         "    name = 'pyruntime',",
         "    interpreter = ':intr',",
         "    files = ['data.txt'],",
+        "    python_version = 'PY2',",
         ")",
         "userruntime(",
         "    name = 'userruntime',",