diff --git a/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java b/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java
index 97fffd3..b476a58 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/RuleClass.java
@@ -134,6 +134,7 @@
       Functions.<Set<String>>constant(ImmutableSet.<String>of());
 
   public static final PathFragment THIRD_PARTY_PREFIX = PathFragment.create("third_party");
+  public static final PathFragment EXPERIMENTAL_PREFIX = PathFragment.create("experimental");
   public static final String EXEC_COMPATIBLE_WITH_ATTR = "exec_compatible_with";
 
   /**
diff --git a/src/main/java/com/google/devtools/build/lib/rules/android/AndroidBinary.java b/src/main/java/com/google/devtools/build/lib/rules/android/AndroidBinary.java
index 4163b1b..d3b5ba0 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/android/AndroidBinary.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/android/AndroidBinary.java
@@ -59,6 +59,7 @@
 import com.google.devtools.build.lib.collect.nestedset.NestedSetBuilder;
 import com.google.devtools.build.lib.concurrent.ThreadSafety.Immutable;
 import com.google.devtools.build.lib.packages.BuildType;
+import com.google.devtools.build.lib.packages.RuleClass;
 import com.google.devtools.build.lib.packages.RuleErrorConsumer;
 import com.google.devtools.build.lib.packages.TriState;
 import com.google.devtools.build.lib.rules.android.AndroidBinaryMobileInstall.MobileInstallResourceApks;
@@ -155,6 +156,34 @@
       ruleContext.throwWithAttributeError(
           "multidex", "Support for Java 8 libraries on legacy devices requires multidex");
     }
+
+    if (ruleContext.getFragment(JavaConfiguration.class).enforceProguardFileExtension()
+        && ruleContext.attributes().has(ProguardHelper.PROGUARD_SPECS)) {
+      List<PathFragment> pathsWithUnexpectedExtension =
+          ruleContext
+              .getPrerequisiteArtifacts(ProguardHelper.PROGUARD_SPECS, Mode.TARGET)
+              .list()
+              .stream()
+              .filter(Artifact::isSourceArtifact)
+              .map(Artifact::getRootRelativePath)
+              .filter(
+                  // This checks the filename directly instead of using FileType because we want to
+                  // exclude third_party/, but FileType is generally only given the basename.
+                  //
+                  // See e.g. RuleContext#validateDirectPrerequisiteType and
+                  // PrerequisiteArtifacts#filter.
+                  path ->
+                      !path.getFileExtension().equals("pgcfg")
+                          && !path.startsWith(RuleClass.THIRD_PARTY_PREFIX)
+                          && !path.startsWith(RuleClass.EXPERIMENTAL_PREFIX))
+              .collect(toImmutableList());
+      if (!pathsWithUnexpectedExtension.isEmpty()) {
+        ruleContext.throwWithAttributeError(
+            ProguardHelper.PROGUARD_SPECS,
+            "Proguard spec files must use the .pgcfg extension. These files do not end in .pgcfg: "
+                + pathsWithUnexpectedExtension);
+      }
+    }
   }
 
   private static RuleConfiguredTargetBuilder init(
diff --git a/src/main/java/com/google/devtools/build/lib/rules/java/JavaConfiguration.java b/src/main/java/com/google/devtools/build/lib/rules/java/JavaConfiguration.java
index 0d8f49f..6fac9a7 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/java/JavaConfiguration.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/java/JavaConfiguration.java
@@ -168,6 +168,7 @@
   private final ImmutableList<Label> translationTargets;
   private final JavaOptimizationMode javaOptimizationMode;
   private final ImmutableMap<String, Optional<Label>> bytecodeOptimizers;
+  private final boolean enforceProguardFileExtension;
   private final Label toolchainLabel;
   private final Label runtimeLabel;
   private final boolean explicitJavaTestDeps;
@@ -199,6 +200,7 @@
     this.fixDepsTool = javaOptions.fixDepsTool;
     this.proguardBinary = javaOptions.proguard;
     this.extraProguardSpecs = ImmutableList.copyOf(javaOptions.extraProguardSpecs);
+    this.enforceProguardFileExtension = javaOptions.enforceProguardFileExtension;
     this.bundleTranslations = javaOptions.bundleTranslations;
     this.toolchainLabel = javaOptions.javaToolchain;
     this.runtimeLabel = javaOptions.javaBase;
@@ -369,6 +371,11 @@
     return extraProguardSpecs;
   }
 
+  /** Returns whether ProGuard configuration files are required to use a *.pgcfg extension. */
+  public boolean enforceProguardFileExtension() {
+    return enforceProguardFileExtension;
+  }
+
   /** Returns the raw translation targets. */
   public ImmutableList<Label> getTranslationTargets() {
     return translationTargets;
diff --git a/src/main/java/com/google/devtools/build/lib/rules/java/JavaOptions.java b/src/main/java/com/google/devtools/build/lib/rules/java/JavaOptions.java
index ebd0156..998e954 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/java/JavaOptions.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/java/JavaOptions.java
@@ -419,6 +419,16 @@
   public Map<String, Label> bytecodeOptimizers;
 
   @Option(
+      name = "enforce_proguard_file_extension",
+      defaultValue = "false",
+      documentationCategory = OptionDocumentationCategory.UNDOCUMENTED,
+      effectTags = {OptionEffectTag.EAGERNESS_TO_EXIT},
+      help =
+          "If enabled, requires that ProGuard configuration files outside of third_party/ use the"
+              + " *.pgcfg file extension.")
+  public boolean enforceProguardFileExtension;
+
+  @Option(
       name = "translations",
       defaultValue = "auto",
       documentationCategory = OptionDocumentationCategory.UNCATEGORIZED,
diff --git a/src/test/java/com/google/devtools/build/lib/rules/android/AndroidBinaryTest.java b/src/test/java/com/google/devtools/build/lib/rules/android/AndroidBinaryTest.java
index e928288..5fbb838 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/android/AndroidBinaryTest.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/android/AndroidBinaryTest.java
@@ -1056,6 +1056,63 @@
   }
 
   @Test
+  public void enforceProguardFileExtension_disabled_allowsOtherExtensions() throws Exception {
+    useConfiguration("--noenforce_proguard_file_extension");
+    scratch.file(
+        "java/android/hello/BUILD",
+        "android_binary(name = 'b',",
+        "               srcs = ['HelloApp.java'],",
+        "               manifest = 'AndroidManifest.xml',",
+        "               proguard_specs = ['proguard-spec.pro'])");
+
+    ConfiguredTarget unused = getConfiguredTarget("//java/android/hello:b");
+  }
+
+  @Test
+  public void enforceProguardFileExtension_enabled_disallowsOtherExtensions() throws Exception {
+    useConfiguration("--enforce_proguard_file_extension");
+    scratch.file(
+        "java/android/hello/BUILD",
+        "android_binary(name = 'b',",
+        "               srcs = ['HelloApp.java'],",
+        "               manifest = 'AndroidManifest.xml',",
+        "               proguard_specs = ['proguard-spec.pro'])");
+
+    AssertionError assertionError =
+        assertThrows(AssertionError.class, () -> getConfiguredTarget("//java/android/hello:b"));
+
+    assertThat(assertionError).hasMessageThat().contains("These files do not end in .pgcfg");
+  }
+
+  @Test
+  public void enforceProguardFileExtension_enabled_allowsPgcfg() throws Exception {
+    useConfiguration("--enforce_proguard_file_extension");
+    scratch.file(
+        "java/android/hello/BUILD",
+        "android_binary(name = 'b',",
+        "               srcs = ['HelloApp.java'],",
+        "               manifest = 'AndroidManifest.xml',",
+        "               proguard_specs = [':proguard.pgcfg'])");
+
+    ConfiguredTarget unused = getConfiguredTarget("//java/android/hello:b");
+  }
+
+  @Test
+  public void enforceProguardFileExtension_enabled_ignoresThirdParty() throws Exception {
+    useConfiguration("--enforce_proguard_file_extension");
+    scratch.file(
+        "third_party/bar/BUILD", "licenses(['unencumbered'])", "exports_files(['proguard.pro'])");
+    scratch.file(
+        "java/android/hello/BUILD",
+        "android_binary(name = 'b',",
+        "               srcs = ['HelloApp.java'],",
+        "               manifest = 'AndroidManifest.xml',",
+        "               proguard_specs = ['//third_party/bar:proguard.pro'])");
+
+    ConfiguredTarget unused = getConfiguredTarget("//java/android/hello:b");
+  }
+
+  @Test
   public void testNeverlinkTransitivity() throws Exception {
     useConfiguration("--android_fixed_resource_neverlinking");