diff --git a/src/main/java/com/google/devtools/build/lib/actions/cache/CompactPersistentActionCache.java b/src/main/java/com/google/devtools/build/lib/actions/cache/CompactPersistentActionCache.java
index 8c936db5eb7059..810fa7c5b33f95 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/cache/CompactPersistentActionCache.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/cache/CompactPersistentActionCache.java
@@ -18,6 +18,7 @@
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Maps;
 import com.google.common.flogger.GoogleLogger;
 import com.google.devtools.build.lib.actions.FileArtifactValue.RemoteFileArtifactValue;
@@ -253,9 +254,37 @@ private static CompactPersistentActionCache create(
       }
       misses.put(reason, new AtomicInteger(0));
     }
+    deleteUnrecognizedFiles(cacheRoot);
     return new CompactPersistentActionCache(indexer, map, Maps.immutableEnumMap(misses));
   }
 
+  /**
+   * Deletes unrecognized files from the action cache on-disk directory.
+   *
+   * <p>This works around an incrementality bug when building a runfiles tree while alternating
+   * between Bazel versions. Specifically, because the runfiles output manifest is a symlink to the
+   * input manifest (and therefore always appears to have the contents of the latter), one can get a
+   * spurious cache hit for a stale runfiles tree if the tree was updated in an intervening build
+   * without the action cache being updated accordingly.
+   *
+   * <p>Backported from https://github.com/bazelbuild/bazel/pull/27525 (Bazel 9.0.0).
+   */
+  private static void deleteUnrecognizedFiles(Path cacheRoot) throws IOException {
+    Path indexFile = cacheRoot.getChild("filename_index_v" + VERSION + ".blaze");
+    Path indexJournalFile = cacheRoot.getChild("filename_index_v" + VERSION + ".journal");
+    ImmutableSet<Path> knownFiles =
+        ImmutableSet.of(
+            cacheFile(cacheRoot),
+            journalFile(cacheRoot),
+            indexFile,
+            indexJournalFile);
+    for (Path child : cacheRoot.getDirectoryEntries()) {
+      if (!knownFiles.contains(child)) {
+        child.delete();
+      }
+    }
+  }
+
   private static CompactPersistentActionCache logAndThrowOrRecurse(
       Path cacheRoot,
       Clock clock,
diff --git a/src/test/java/com/google/devtools/build/lib/actions/cache/CompactPersistentActionCacheTest.java b/src/test/java/com/google/devtools/build/lib/actions/cache/CompactPersistentActionCacheTest.java
index 838af5eda54dee..3c6eafd6ccb347 100644
--- a/src/test/java/com/google/devtools/build/lib/actions/cache/CompactPersistentActionCacheTest.java
+++ b/src/test/java/com/google/devtools/build/lib/actions/cache/CompactPersistentActionCacheTest.java
@@ -451,6 +451,15 @@ public void putAndGet_treeMetadata_savesMaterializationExecPath() {
                 Optional.of(materializationExecPath)));
   }
 
+  @Test
+  public void testDeleteUnrecognizedFiles() throws Exception {
+    Path unrecognizedFile = dataRoot.getChild("unrecognized_file");
+    FileSystemUtils.writeContentAsLatin1(unrecognizedFile, "content");
+    assertThat(unrecognizedFile.exists()).isTrue();
+    CompactPersistentActionCache.create(dataRoot, clock, NullEventHandler.INSTANCE);
+    assertThat(unrecognizedFile.exists()).isFalse();
+  }
+
   private static void assertKeyEquals(ActionCache cache1, ActionCache cache2, String key) {
     Object entry = cache1.get(key);
     assertThat(entry).isNotNull();