diff --git a/solr/CHANGES.txt b/solr/CHANGES.txt
index 518f63a10d9..b2a384b2ce4 100644
--- a/solr/CHANGES.txt
+++ b/solr/CHANGES.txt
@@ -203,6 +203,9 @@ Bug Fixes
 
 * SOLR-9397: Config API does not support adding caches (noble)
 
+* SOLR-9405: ConcurrentModificationException in ZkStateReader.getStateWatchers.
+  (Alan Woodward, Edward Ribeiro, shalin)
+
 Optimizations
 ----------------------
 
diff --git a/solr/solrj/src/java/org/apache/solr/common/cloud/ZkStateReader.java b/solr/solrj/src/java/org/apache/solr/common/cloud/ZkStateReader.java
index 9df4a76aa8e..a3de324e0a7 100644
--- a/solr/solrj/src/java/org/apache/solr/common/cloud/ZkStateReader.java
+++ b/solr/solrj/src/java/org/apache/solr/common/cloud/ZkStateReader.java
@@ -147,7 +147,7 @@ public class ZkStateReader implements Closeable {
   private class CollectionWatch {
 
     int coreRefCount = 0;
-    Set<CollectionStateWatcher> stateWatchers = new HashSet<>();
+    Set<CollectionStateWatcher> stateWatchers = ConcurrentHashMap.newKeySet();
 
     public boolean canBeRemoved() {
       return coreRefCount + stateWatchers.size() == 0;
@@ -1273,10 +1273,14 @@ public class ZkStateReader implements Closeable {
 
   /* package-private for testing */
   Set<CollectionStateWatcher> getStateWatchers(String collection) {
-    CollectionWatch watch = collectionWatches.get(collection);
-    if (watch == null)
-      return null;
-    return new HashSet<>(watch.stateWatchers);
+    final Set<CollectionStateWatcher> watchers = new HashSet<>();
+    collectionWatches.compute(collection, (k, v) -> {
+      if (v != null) {
+        watchers.addAll(v.stateWatchers);
+      }
+      return v;
+    });
+    return watchers;
   }
 
   // returns true if the state has changed
diff --git a/solr/solrj/src/test/org/apache/solr/common/cloud/TestCollectionStateWatchers.java b/solr/solrj/src/test/org/apache/solr/common/cloud/TestCollectionStateWatchers.java
index d959aa83c9b..fca0e35728c 100644
--- a/solr/solrj/src/test/org/apache/solr/common/cloud/TestCollectionStateWatchers.java
+++ b/solr/solrj/src/test/org/apache/solr/common/cloud/TestCollectionStateWatchers.java
@@ -19,8 +19,9 @@ package org.apache.solr.common.cloud;
 
 import java.lang.invoke.MethodHandles;
 import java.util.HashMap;
-import java.util.Set;
+import java.util.concurrent.Callable;
 import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Future;
 import java.util.concurrent.TimeUnit;
@@ -81,6 +82,31 @@ public class TestCollectionStateWatchers extends SolrCloudTestCase {
     });
   }
 
+  private static void waitFor(String message, long timeout, TimeUnit unit, Callable<Boolean> predicate)
+      throws InterruptedException, ExecutionException {
+    Future<Boolean> future = executor.submit(() -> {
+      try {
+        while (true) {
+          if (predicate.call())
+            return true;
+          TimeUnit.MILLISECONDS.sleep(10);
+        }
+      }
+      catch (InterruptedException e) {
+        return false;
+      }
+    });
+    try {
+      if (future.get(timeout, unit) == true) {
+        return;
+      }
+    }
+    catch (TimeoutException e) {
+      // pass failure message on
+    }
+    future.cancel(true);
+    fail(message);
+  }
 
   @Test
   public void testSimpleCollectionWatch() throws Exception {
@@ -113,9 +139,8 @@ public class TestCollectionStateWatchers extends SolrCloudTestCase {
     cluster.stopJettySolrRunner(random().nextInt(cluster.getJettySolrRunners().size()));
     assertTrue("CollectionStateWatcher was never notified of cluster change", latch.await(MAX_WAIT_TIMEOUT, TimeUnit.SECONDS));
 
-    Set<CollectionStateWatcher> watchers = client.getZkStateReader().getStateWatchers("testcollection");
-    assertTrue("CollectionStateWatcher wasn't cleared after completion",
-        watchers == null || watchers.size() == 0);
+    waitFor("CollectionStateWatcher wasn't cleared after completion", 1, TimeUnit.SECONDS,
+        () -> client.getZkStateReader().getStateWatchers("testcollection").isEmpty());
 
   }
 
@@ -144,8 +169,8 @@ public class TestCollectionStateWatchers extends SolrCloudTestCase {
 
     assertTrue("CollectionStateWatcher isn't called when registering for already-watched collection",
         latch.await(MAX_WAIT_TIMEOUT, TimeUnit.SECONDS));
-    assertEquals("CollectionStateWatcher should be removed",
-        1, client.getZkStateReader().getStateWatchers("currentstate").size());
+    waitFor("CollectionStateWatcher should be removed", 1, TimeUnit.SECONDS,
+        () -> client.getZkStateReader().getStateWatchers("currentstate").size() == 1);
   }
 
   @Test
@@ -189,9 +214,8 @@ public class TestCollectionStateWatchers extends SolrCloudTestCase {
     expectThrows(TimeoutException.class, () -> {
       client.waitForState("nosuchcollection", 1, TimeUnit.SECONDS, ((liveNodes, collectionState) -> false));
     });
-    Set<CollectionStateWatcher> watchers = client.getZkStateReader().getStateWatchers("nosuchcollection");
-    assertTrue("Watchers for collection should be removed after timeout",
-        watchers == null || watchers.size() == 0);
+    waitFor("Watchers for collection should be removed after timeout", 1, TimeUnit.SECONDS,
+        () -> client.getZkStateReader().getStateWatchers("nosuchcollection").isEmpty());
 
   }
 
@@ -229,18 +253,17 @@ public class TestCollectionStateWatchers extends SolrCloudTestCase {
   }
 
   @Test
-  public void testWatcherIsRemovedAfterTimeout() {
+  public void testWatcherIsRemovedAfterTimeout() throws Exception {
     CloudSolrClient client = cluster.getSolrClient();
     assertTrue("There should be no watchers for a non-existent collection!",
-        client.getZkStateReader().getStateWatchers("no-such-collection") == null);
+        client.getZkStateReader().getStateWatchers("no-such-collection").isEmpty());
 
     expectThrows(TimeoutException.class, () -> {
       client.waitForState("no-such-collection", 10, TimeUnit.MILLISECONDS, (n, c) -> DocCollection.isFullyActive(n, c, 1, 1));
     });
 
-    Set<CollectionStateWatcher> watchers = client.getZkStateReader().getStateWatchers("no-such-collection");
-    assertTrue("Watchers for collection should be removed after timeout",
-        watchers == null || watchers.size() == 0);
+    waitFor("Watchers for collection should be removed after timeout", 1, TimeUnit.SECONDS,
+        () -> client.getZkStateReader().getStateWatchers("no-such-collection").isEmpty());
 
   }