diff --git a/src/main/java/com/google/devtools/build/lib/cmdline/RepositoryMapping.java b/src/main/java/com/google/devtools/build/lib/cmdline/RepositoryMapping.java
index 0246c930171776..d1e5f35cc7c53f 100644
--- a/src/main/java/com/google/devtools/build/lib/cmdline/RepositoryMapping.java
+++ b/src/main/java/com/google/devtools/build/lib/cmdline/RepositoryMapping.java
@@ -17,6 +17,8 @@
 import com.google.auto.value.AutoValue;
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.Interner;
+import com.google.devtools.build.lib.concurrent.BlazeInterners;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -34,6 +36,15 @@
 @AutoValue
 public abstract class RepositoryMapping {
 
+  // All repositories generated by a module extension have the same repository mapping
+  // entries. Without interning, if a module extension generates N repositories, each such
+  // repository would have its own copy with (more than) N entries, resulting in memory usage that
+  // is quadratic in N.
+  // Note: We intern the entries, not the RepositoryMapping itself, because the latter also includes
+  // the owner repo, which differs between extension repos.
+  private static final Interner<ImmutableMap<String, RepositoryName>> ENTRIES_INTERNER =
+      BlazeInterners.newWeakInterner();
+
   // Always fallback to the requested name
   public static final RepositoryMapping ALWAYS_FALLBACK = createAllowingFallback(ImmutableMap.of());
 
@@ -60,7 +71,8 @@ public static RepositoryMapping createAllowingFallback(Map<String, RepositoryNam
 
   private static RepositoryMapping createInternal(
       Map<String, RepositoryName> entries, RepositoryName ownerRepo) {
-    return new AutoValue_RepositoryMapping(ImmutableMap.copyOf(entries), ownerRepo);
+    return new AutoValue_RepositoryMapping(
+        ENTRIES_INTERNER.intern(ImmutableMap.copyOf(entries)), ownerRepo);
   }
 
   /**