diff --git a/docs/changelog/131429.yaml b/docs/changelog/131429.yaml
new file mode 100644
index 0000000000000..8ffb7d2ffe100
--- /dev/null
+++ b/docs/changelog/131429.yaml
@@ -0,0 +1,5 @@
+pr: 131429
+summary: Prevent auto-sharding for data streams in LOOKUP index mode
+area: Data streams
+type: bug
+issues: []
diff --git a/server/src/main/java/org/elasticsearch/action/datastreams/autosharding/DataStreamAutoShardingService.java b/server/src/main/java/org/elasticsearch/action/datastreams/autosharding/DataStreamAutoShardingService.java
index 61eb2867790a2..b5c884be746dc 100644
--- a/server/src/main/java/org/elasticsearch/action/datastreams/autosharding/DataStreamAutoShardingService.java
+++ b/server/src/main/java/org/elasticsearch/action/datastreams/autosharding/DataStreamAutoShardingService.java
@@ -26,6 +26,7 @@
 import org.elasticsearch.core.Nullable;
 import org.elasticsearch.core.TimeValue;
 import org.elasticsearch.index.Index;
+import org.elasticsearch.index.IndexMode;
 import org.elasticsearch.index.shard.IndexingStats;
 
 import java.util.Arrays;
@@ -371,6 +372,7 @@ public String toString() {
      * there'll be no new auto sharding event)
      */
     public AutoShardingResult calculate(ProjectState state, DataStream dataStream, @Nullable IndexStats writeIndexStats) {
+
         if (isAutoShardingEnabled == false) {
             logger.debug("Data stream auto-sharding service is not enabled.");
             return NOT_APPLICABLE_RESULT;
@@ -385,6 +387,11 @@ public AutoShardingResult calculate(ProjectState state, DataStream dataStream, @
             return NOT_APPLICABLE_RESULT;
         }
 
+        if (dataStream.getIndexMode() == IndexMode.LOOKUP) {
+            logger.debug("Data stream [{}] has indexing mode LOOKUP; auto-sharding is not applicable.", dataStream.getName());
+            return NOT_APPLICABLE_RESULT;
+        }
+
         if (writeIndexStats == null) {
             logger.debug(
                 "Data stream auto-sharding service cannot compute the optimal number of shards for data stream [{}] as the write index "
diff --git a/server/src/test/java/org/elasticsearch/action/datastreams/autosharding/DataStreamAutoShardingServiceTests.java b/server/src/test/java/org/elasticsearch/action/datastreams/autosharding/DataStreamAutoShardingServiceTests.java
index 403c395e31b57..0fa419be7b60b 100644
--- a/server/src/test/java/org/elasticsearch/action/datastreams/autosharding/DataStreamAutoShardingServiceTests.java
+++ b/server/src/test/java/org/elasticsearch/action/datastreams/autosharding/DataStreamAutoShardingServiceTests.java
@@ -1343,4 +1343,46 @@ private static Decision.Inputs createDecisionInputsForPeriodLoggerTests(int writ
             3
         );
     }
+
+    public void testCalculateReturnsNotApplicableForLookupIndexMode() {
+        var projectId = randomProjectIdOrDefault();
+        ProjectMetadata.Builder builder = ProjectMetadata.builder(projectId);
+        DataStream dataStream = createLookupModeDataStream(builder);
+        ClusterState state = createClusterStateWithDataStream(builder);
+
+        AutoShardingResult autoShardingResult = service.calculate(
+            state.projectState(projectId),
+            dataStream,
+            createIndexStats(1, 1.0, 1.0, 1.0)
+        );
+        assertThat(autoShardingResult, is(NOT_APPLICABLE_RESULT));
+        assertThat(decisionsLogged, hasSize(0));
+    }
+
+    public void testCalculateReturnsNotApplicableForLookupIndexModeWithNullStats() {
+        var projectId = randomProjectIdOrDefault();
+        ProjectMetadata.Builder builder = ProjectMetadata.builder(projectId);
+        DataStream dataStream = createLookupModeDataStream(builder);
+        ClusterState state = createClusterStateWithDataStream(builder);
+
+        AutoShardingResult autoShardingResult = service.calculate(state.projectState(projectId), dataStream, null);
+        assertThat(autoShardingResult, is(NOT_APPLICABLE_RESULT));
+        assertThat(decisionsLogged, hasSize(0));
+    }
+
+    private DataStream createLookupModeDataStream(ProjectMetadata.Builder builder) {
+        DataStream dataStream = DataStream.builder(dataStreamName, List.of(new Index("test-index", randomUUID())))
+            .setGeneration(1)
+            .setIndexMode(IndexMode.LOOKUP)
+            .build();
+        builder.put(dataStream);
+        return dataStream;
+    }
+
+    private ClusterState createClusterStateWithDataStream(ProjectMetadata.Builder builder) {
+        return ClusterState.builder(ClusterName.DEFAULT)
+            .nodes(DiscoveryNodes.builder().add(DiscoveryNodeUtils.create("n1")))
+            .putProjectMetadata(builder.build())
+            .build();
+    }
 }