diff --git a/server/src/main/java/org/elasticsearch/cluster/metadata/MetadataCreateIndexService.java b/server/src/main/java/org/elasticsearch/cluster/metadata/MetadataCreateIndexService.java
index 32ec46b0badc6..7375b03306da2 100644
--- a/server/src/main/java/org/elasticsearch/cluster/metadata/MetadataCreateIndexService.java
+++ b/server/src/main/java/org/elasticsearch/cluster/metadata/MetadataCreateIndexService.java
@@ -857,7 +857,9 @@ static int getIndexNumberOfRoutingShards(Settings indexSettings, @Nullable Index
             // in this case we either have no index to recover from or
             // we have a source index with 1 shard and without an explicit split factor
             // or one that is valid in that case we can split into whatever and auto-generate a new factor.
-            if (IndexMetadata.INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.exists(indexSettings)) {
+            // (Don't use IndexMetadata.INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.get(indexSettings) here, otherwise
+            // we get the default value when `null` has been provided as value)
+            if (indexSettings.get(IndexMetadata.INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.getKey()) != null) {
                 routingNumShards = IndexMetadata.INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.get(indexSettings);
             } else {
                 routingNumShards = calculateNumRoutingShards(numTargetShards, indexVersionCreated);
diff --git a/server/src/test/java/org/elasticsearch/cluster/metadata/MetadataCreateIndexServiceTests.java b/server/src/test/java/org/elasticsearch/cluster/metadata/MetadataCreateIndexServiceTests.java
index eae70ed1b81cd..269ae0f6d7754 100644
--- a/server/src/test/java/org/elasticsearch/cluster/metadata/MetadataCreateIndexServiceTests.java
+++ b/server/src/test/java/org/elasticsearch/cluster/metadata/MetadataCreateIndexServiceTests.java
@@ -935,6 +935,29 @@ public void testGetIndexNumberOfRoutingShardsWhenExplicitlyConfigured() {
         assertThat(targetRoutingNumberOfShards, is(9));
     }
 
+    public void testGetIndexNumberOfRoutingShardsNullVsNotDefined() {
+        int numberOfPrimaryShards = randomIntBetween(1, 16);
+        Settings indexSettings = settings(Version.CURRENT)
+            .put(INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.getKey(), (String) null)
+            .put(INDEX_NUMBER_OF_SHARDS_SETTING.getKey(), numberOfPrimaryShards)
+            .build();
+        int targetRoutingNumberOfShardsWithNull = getIndexNumberOfRoutingShards(indexSettings, null);
+        indexSettings = settings(Version.CURRENT)
+            .put(INDEX_NUMBER_OF_SHARDS_SETTING.getKey(), numberOfPrimaryShards)
+            .build();
+        int targetRoutingNumberOfShardsWithNotDefined = getIndexNumberOfRoutingShards(indexSettings, null);
+        assertThat(targetRoutingNumberOfShardsWithNull, is(targetRoutingNumberOfShardsWithNotDefined));
+    }
+
+    public void testGetIndexNumberOfRoutingShardsNull() {
+        Settings indexSettings = settings(Version.CURRENT)
+            .put(INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.getKey(), (String) null)
+            .put(INDEX_NUMBER_OF_SHARDS_SETTING.getKey(), 2)
+            .build();
+        int targetRoutingNumberOfShardsWithNull = getIndexNumberOfRoutingShards(indexSettings, null);
+        assertThat(targetRoutingNumberOfShardsWithNull, is(1024));
+    }
+
     public void testGetIndexNumberOfRoutingShardsYieldsSourceNumberOfShards() {
         Settings indexSettings = Settings.builder()
             .put(INDEX_NUMBER_OF_SHARDS_SETTING.getKey(), 3)