diff --git a/docs/changelog/126077.yaml b/docs/changelog/126077.yaml
new file mode 100644
index 0000000000000..ffb98d71ba630
--- /dev/null
+++ b/docs/changelog/126077.yaml
@@ -0,0 +1,6 @@
+pr: 126077
+summary: Preventing `ConcurrentModificationException` when updating settings for more
+  than one index
+area: Indices APIs
+type: bug
+issues: []
diff --git a/server/src/internalClusterTest/java/org/elasticsearch/cluster/metadata/MetadataUpdateSettingsServiceIT.java b/server/src/internalClusterTest/java/org/elasticsearch/cluster/metadata/MetadataUpdateSettingsServiceIT.java
index 1d95f3061abde..4f4dfda343cae 100644
--- a/server/src/internalClusterTest/java/org/elasticsearch/cluster/metadata/MetadataUpdateSettingsServiceIT.java
+++ b/server/src/internalClusterTest/java/org/elasticsearch/cluster/metadata/MetadataUpdateSettingsServiceIT.java
@@ -39,7 +39,8 @@ public void testThatNonDynamicSettingChangesTakeEffect() throws Exception {
          * This test makes sure that when non-dynamic settings are updated that they actually take effect (as opposed to just being set
          * in the cluster state).
          */
-        createIndex("test", Settings.EMPTY);
+        createIndex("test-1", Settings.EMPTY);
+        createIndex("test-2", Settings.EMPTY);
         MetadataUpdateSettingsService metadataUpdateSettingsService = internalCluster().getCurrentMasterNodeInstance(
             MetadataUpdateSettingsService.class
         );
diff --git a/server/src/main/java/org/elasticsearch/cluster/metadata/MetadataUpdateSettingsService.java b/server/src/main/java/org/elasticsearch/cluster/metadata/MetadataUpdateSettingsService.java
index 8a2ae77c761dc..db6f6b0139bc1 100644
--- a/server/src/main/java/org/elasticsearch/cluster/metadata/MetadataUpdateSettingsService.java
+++ b/server/src/main/java/org/elasticsearch/cluster/metadata/MetadataUpdateSettingsService.java
@@ -219,7 +219,7 @@ ClusterState execute(ClusterState currentState) {
                     // We have non-dynamic settings and open indices. We will unassign all of the shards in these indices so that the new
                     // changed settings are applied when the shards are re-assigned.
                     routingTableBuilder = RoutingTable.builder(allocationService.getShardRoutingRoleStrategy(), currentRoutingTable);
-                    for (Index index : openIndices) {
+                    for (Index index : new HashSet<>(openIndices)) {
                         // We only want to take on the expense of reopening all shards for an index if the setting is really changing
                         Settings existingSettings = currentProject.index(index).getSettings();
                         boolean needToReopenIndex = false;