diff --git a/server/src/main/java/org/elasticsearch/index/engine/ReadOnlyEngine.java b/server/src/main/java/org/elasticsearch/index/engine/ReadOnlyEngine.java
index 2b16312b5dca6..6fb41e5da45bf 100644
--- a/server/src/main/java/org/elasticsearch/index/engine/ReadOnlyEngine.java
+++ b/server/src/main/java/org/elasticsearch/index/engine/ReadOnlyEngine.java
@@ -29,6 +29,7 @@
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.store.Lock;
 import org.elasticsearch.Version;
+import org.elasticsearch.action.admin.indices.forcemerge.ForceMergeRequest;
 import org.elasticsearch.common.lucene.Lucene;
 import org.elasticsearch.common.lucene.index.ElasticsearchDirectoryReader;
 import org.elasticsearch.common.util.concurrent.ReleasableLock;
@@ -375,6 +376,16 @@ public void flush(boolean force, boolean waitIfOngoing) throws EngineException {
     @Override
     public void forceMerge(boolean flush, int maxNumSegments, boolean onlyExpungeDeletes,
                            boolean upgrade, boolean upgradeOnlyAncientSegments, String forceMergeUUID) {
+        if (maxNumSegments == ForceMergeRequest.Defaults.MAX_NUM_SEGMENTS) {
+            // noop
+        } else if (maxNumSegments < lastCommittedSegmentInfos.size()) {
+            throw new UnsupportedOperationException("force merge is not supported on a read-only engine, " +
+                "target max number of segments[" + maxNumSegments + "], " +
+                "current number of segments[" + lastCommittedSegmentInfos.size() + "].");
+        } else {
+            logger.debug("current number of segments[{}] is not greater than target max number of segments[{}].",
+                lastCommittedSegmentInfos.size(), maxNumSegments);
+        }
     }
 
     @Override
diff --git a/server/src/test/java/org/elasticsearch/index/engine/ReadOnlyEngineTests.java b/server/src/test/java/org/elasticsearch/index/engine/ReadOnlyEngineTests.java
index f52461fe990d8..71d46db74d0a1 100644
--- a/server/src/test/java/org/elasticsearch/index/engine/ReadOnlyEngineTests.java
+++ b/server/src/test/java/org/elasticsearch/index/engine/ReadOnlyEngineTests.java
@@ -21,6 +21,8 @@
 import org.apache.lucene.index.DirectoryReader;
 import org.apache.lucene.index.IndexReader;
 import org.apache.lucene.util.LuceneTestCase;
+import org.elasticsearch.action.admin.indices.forcemerge.ForceMergeRequest;
+import org.elasticsearch.common.UUIDs;
 import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.lucene.index.ElasticsearchDirectoryReader;
 import org.elasticsearch.core.internal.io.IOUtils;
@@ -185,6 +187,43 @@ public void testVerifyShardBeforeIndexClosingIsNoOp() throws IOException {
         }
     }
 
+    public void testForceMergeOnReadOnlyEngine() throws IOException {
+        IOUtils.close(engine, store);
+        final AtomicLong globalCheckpoint = new AtomicLong(SequenceNumbers.NO_OPS_PERFORMED);
+        try (Store store = createStore()) {
+            EngineConfig config = config(defaultSettings, store, createTempDir(), newMergePolicy(), null, null, globalCheckpoint::get);
+            int numDocs = scaledRandomIntBetween(10, 100);
+            int numSegments;
+            try (InternalEngine engine = createEngine(config)) {
+                for (int i = 0; i < numDocs; i++) {
+                    ParsedDocument doc = testParsedDocument(Integer.toString(i), null, testDocument(), new BytesArray("{}"), null);
+                    engine.index(new Engine.Index(newUid(doc), doc, i, primaryTerm.get(), 1, null, Engine.Operation.Origin.REPLICA,
+                        System.nanoTime(), -1, false, SequenceNumbers.UNASSIGNED_SEQ_NO, 0));
+                    engine.flush();
+                    globalCheckpoint.set(i);
+                }
+                engine.syncTranslog();
+                engine.flushAndClose();
+                numSegments = engine.getLastCommittedSegmentInfos().size();
+                assertTrue( numSegments > 1);
+            }
+
+            try (ReadOnlyEngine readOnlyEngine = new ReadOnlyEngine(config, null , null, true, Function.identity(), true)) {
+                UnsupportedOperationException exception = expectThrows(UnsupportedOperationException.class,
+                    () -> readOnlyEngine.forceMerge(
+                        true, numSegments-1, false, false, false, UUIDs.randomBase64UUID()));
+                assertThat(exception.getMessage(), equalTo("force merge is not supported on a read-only engine, " +
+                    "target max number of segments[" + (numSegments-1) + "], current number of segments[" + numSegments + "]."));
+
+                readOnlyEngine.forceMerge(true, ForceMergeRequest.Defaults.MAX_NUM_SEGMENTS,
+                    false, false, false, UUIDs.randomBase64UUID());
+                readOnlyEngine.forceMerge(true, numSegments, false, false, false, UUIDs.randomBase64UUID());
+                readOnlyEngine.forceMerge(true, numSegments+1, false, false, false, UUIDs.randomBase64UUID());
+                assertEquals(readOnlyEngine.getLastCommittedSegmentInfos().size(), numSegments);
+            }
+        }
+    }
+
     public void testRecoverFromTranslogAppliesNoOperations() throws IOException {
         IOUtils.close(engine, store);
         final AtomicLong globalCheckpoint = new AtomicLong(SequenceNumbers.NO_OPS_PERFORMED);