diff --git a/docs/changelog/129945.yaml b/docs/changelog/129945.yaml
new file mode 100644
index 0000000000000..8271c77c08ab1
--- /dev/null
+++ b/docs/changelog/129945.yaml
@@ -0,0 +1,5 @@
+pr: 129945
+summary: "Enhancement: ILM sets `indexing_complete` to true from `ReadOnly` action"
+area: ILM+SLM
+type: enhancement
+issues: []
diff --git a/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/ReadOnlyAction.java b/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/ReadOnlyAction.java
index b93bf652b84b4..0ce513b782b25 100644
--- a/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/ReadOnlyAction.java
+++ b/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/ReadOnlyAction.java
@@ -10,6 +10,7 @@
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
+import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.xcontent.ObjectParser;
 import org.elasticsearch.xcontent.XContentBuilder;
 import org.elasticsearch.xcontent.XContentParser;
@@ -27,6 +28,10 @@ public class ReadOnlyAction implements LifecycleAction {
 
     private static final ObjectParser<ReadOnlyAction, Void> PARSER = new ObjectParser<>(NAME, false, ReadOnlyAction::new);
 
+    public static final String INDEXING_COMPLETE_STEP_NAME = "set-indexing-complete";
+
+    private static final Settings INDEXING_COMPLETE = Settings.builder().put(LifecycleSettings.LIFECYCLE_INDEXING_COMPLETE, true).build();
+
     public static ReadOnlyAction parse(XContentParser parser) {
         return PARSER.apply(parser, null);
     }
@@ -60,6 +65,8 @@ public List<Step> toSteps(Client client, String phase, StepKey nextStepKey) {
         StepKey checkNotWriteIndex = new StepKey(phase, NAME, CheckNotDataStreamWriteIndexStep.NAME);
         StepKey waitTimeSeriesEndTimePassesKey = new StepKey(phase, NAME, WaitUntilTimeSeriesEndTimePassesStep.NAME);
         StepKey readOnlyKey = new StepKey(phase, NAME, NAME);
+        StepKey setIndexingCompleteStepKey = new StepKey(phase, NAME, INDEXING_COMPLETE_STEP_NAME);
+
         CheckNotDataStreamWriteIndexStep checkNotWriteIndexStep = new CheckNotDataStreamWriteIndexStep(
             checkNotWriteIndex,
             waitTimeSeriesEndTimePassesKey
@@ -69,8 +76,16 @@ public List<Step> toSteps(Client client, String phase, StepKey nextStepKey) {
             readOnlyKey,
             Instant::now
         );
-        ReadOnlyStep readOnlyStep = new ReadOnlyStep(readOnlyKey, nextStepKey, client, true);
-        return List.of(checkNotWriteIndexStep, waitUntilTimeSeriesEndTimeStep, readOnlyStep);
+        ReadOnlyStep readOnlyStep = new ReadOnlyStep(readOnlyKey, setIndexingCompleteStepKey, client, true);
+
+        UpdateSettingsStep setIndexingCompleteStep = new UpdateSettingsStep(
+            setIndexingCompleteStepKey,
+            nextStepKey,
+            client,
+            INDEXING_COMPLETE
+        );
+
+        return List.of(checkNotWriteIndexStep, waitUntilTimeSeriesEndTimeStep, readOnlyStep, setIndexingCompleteStep);
     }
 
     @Override
diff --git a/x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ilm/ReadOnlyActionTests.java b/x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ilm/ReadOnlyActionTests.java
index 81fa437f2768d..abbdc6aca5035 100644
--- a/x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ilm/ReadOnlyActionTests.java
+++ b/x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ilm/ReadOnlyActionTests.java
@@ -12,6 +12,7 @@
 
 import java.util.List;
 
+import static org.elasticsearch.xpack.core.ilm.ReadOnlyAction.INDEXING_COMPLETE_STEP_NAME;
 import static org.hamcrest.Matchers.equalTo;
 
 public class ReadOnlyActionTests extends AbstractActionTestCase<ReadOnlyAction> {
@@ -46,13 +47,16 @@ public void testToSteps() {
         );
         List<Step> steps = action.toSteps(null, phase, nextStepKey);
         assertNotNull(steps);
-        assertEquals(3, steps.size());
+        assertEquals(4, steps.size());
         StepKey expectedFirstStepKey = new StepKey(phase, ReadOnlyAction.NAME, CheckNotDataStreamWriteIndexStep.NAME);
         StepKey expectedSecondStepKey = new StepKey(phase, ReadOnlyAction.NAME, WaitUntilTimeSeriesEndTimePassesStep.NAME);
         StepKey expectedThirdStepKey = new StepKey(phase, ReadOnlyAction.NAME, ReadOnlyAction.NAME);
+        StepKey expectedFourthStepKey = new StepKey(phase, ReadOnlyAction.NAME, INDEXING_COMPLETE_STEP_NAME);
+
         CheckNotDataStreamWriteIndexStep firstStep = (CheckNotDataStreamWriteIndexStep) steps.get(0);
         WaitUntilTimeSeriesEndTimePassesStep secondStep = (WaitUntilTimeSeriesEndTimePassesStep) steps.get(1);
         ReadOnlyStep thirdStep = (ReadOnlyStep) steps.get(2);
+        UpdateSettingsStep fourthStep = (UpdateSettingsStep) steps.get(3);
 
         assertThat(firstStep.getKey(), equalTo(expectedFirstStepKey));
         assertThat(firstStep.getNextStepKey(), equalTo(expectedSecondStepKey));
@@ -61,7 +65,10 @@ public void testToSteps() {
         assertThat(secondStep.getNextStepKey(), equalTo(expectedThirdStepKey));
 
         assertThat(thirdStep.getKey(), equalTo(expectedThirdStepKey));
-        assertThat(thirdStep.getNextStepKey(), equalTo(nextStepKey));
+        assertThat(thirdStep.getNextStepKey(), equalTo(expectedFourthStepKey));
+
+        assertThat(fourthStep.getKey(), equalTo(expectedFourthStepKey));
+        assertThat(fourthStep.getNextStepKey(), equalTo(nextStepKey));
     }
 
 }
diff --git a/x-pack/plugin/ilm/qa/multi-node/src/javaRestTest/java/org/elasticsearch/xpack/ilm/actions/ReadonlyActionIT.java b/x-pack/plugin/ilm/qa/multi-node/src/javaRestTest/java/org/elasticsearch/xpack/ilm/actions/ReadonlyActionIT.java
index ec0b685dd5bd0..3caf53ee3376e 100644
--- a/x-pack/plugin/ilm/qa/multi-node/src/javaRestTest/java/org/elasticsearch/xpack/ilm/actions/ReadonlyActionIT.java
+++ b/x-pack/plugin/ilm/qa/multi-node/src/javaRestTest/java/org/elasticsearch/xpack/ilm/actions/ReadonlyActionIT.java
@@ -64,6 +64,7 @@ public void testReadOnly() throws Exception {
             assertThat(getStepKeyForIndex(client(), index), equalTo(PhaseCompleteStep.finalStep(phaseName).getKey()));
             assertThat(settings.get(IndexMetadata.INDEX_BLOCKS_WRITE_SETTING.getKey()), equalTo("true"));
             assertThat(settings.get(IndexMetadata.INDEX_BLOCKS_METADATA_SETTING.getKey()), nullValue());
+            assertThat(settings.get(LifecycleSettings.LIFECYCLE_INDEXING_COMPLETE_SETTING.getKey()), equalTo("true"));
         });
     }