diff --git a/modules/reindex-management/qa/build.gradle b/modules/reindex-management/qa/build.gradle
new file mode 100644
index 0000000000000..0f98e90b4d52e
--- /dev/null
+++ b/modules/reindex-management/qa/build.gradle
@@ -0,0 +1,9 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
diff --git a/modules/reindex-management/qa/multi-project/build.gradle b/modules/reindex-management/qa/multi-project/build.gradle
new file mode 100644
index 0000000000000..df3dddf6ab47d
--- /dev/null
+++ b/modules/reindex-management/qa/multi-project/build.gradle
@@ -0,0 +1,26 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v 3.0 only", or the "Server Side Public License, v 1".
+ */
+
+apply plugin: 'elasticsearch.internal-java-rest-test'
+
+dependencies {
+  clusterModules project(':modules:reindex')
+  clusterModules project(':modules:reindex-management')
+  clusterModules project(':test:external-modules:test-multi-project')
+}
+
+tasks.withType(Test).configureEach {
+  it.systemProperty "tests.multi_project.enabled", true
+}
+
+// exclude multi-project tests from release build
+tasks.named('javaRestTest') {
+  def isSnapshot = buildParams.snapshotBuild
+  it.onlyIf("snapshot build") { isSnapshot }
+}
diff --git a/modules/reindex-management/qa/multi-project/src/javaRestTest/java/org/elasticsearch/reindex/management/ReindexManagementMultiProjectIT.java b/modules/reindex-management/qa/multi-project/src/javaRestTest/java/org/elasticsearch/reindex/management/ReindexManagementMultiProjectIT.java
new file mode 100644
index 0000000000000..55f838806eb3b
--- /dev/null
+++ b/modules/reindex-management/qa/multi-project/src/javaRestTest/java/org/elasticsearch/reindex/management/ReindexManagementMultiProjectIT.java
@@ -0,0 +1,180 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v 3.0 only", or the "Server Side Public License, v 1".
+ */
+
+package org.elasticsearch.reindex.management;
+
+import org.elasticsearch.client.Request;
+import org.elasticsearch.client.RequestOptions;
+import org.elasticsearch.client.Response;
+import org.elasticsearch.client.ResponseException;
+import org.elasticsearch.common.Strings;
+import org.elasticsearch.tasks.Task;
+import org.elasticsearch.tasks.TaskId;
+import org.elasticsearch.test.cluster.ElasticsearchCluster;
+import org.elasticsearch.test.rest.ESRestTestCase;
+import org.elasticsearch.test.rest.ObjectPath;
+import org.junit.After;
+import org.junit.ClassRule;
+
+import java.io.IOException;
+import java.util.Map;
+
+import static org.hamcrest.Matchers.equalTo;
+
+/** Tests that endpoints in reindex-management module are project-aware and behave as expected in multi-project environments. */
+public class ReindexManagementMultiProjectIT extends ESRestTestCase {
+
+    private static final String SOURCE_INDEX = "reindex_src";
+    private static final String DEST_INDEX = "reindex_dst";
+    private static final int BULK_SIZE = 1;
+    private static final int REQUESTS_PER_SECOND = 1;
+    private static final int NUM_OF_SLICES = 2;
+    private static final int NUMBER_OF_DOCUMENTS_THAT_TAKES_30_SECS_TO_INGEST = 30 * REQUESTS_PER_SECOND * BULK_SIZE;
+
+    @ClassRule
+    public static ElasticsearchCluster cluster = ElasticsearchCluster.local()
+        .nodes(2)   // 2 to test transport serializes projectId over wire.
+        .module("reindex")
+        .module("reindex-management")
+        .module("test-multi-project")
+        .setting("test.multi_project.enabled", "true")
+        .setting("xpack.security.enabled", "false")
+        .setting("xpack.ml.enabled", "false")
+        .build();
+
+    @Override
+    protected boolean shouldConfigureProjects() {
+        return false;
+    }
+
+    @Override
+    protected String getTestRestCluster() {
+        return cluster.getHttpAddresses();
+    }
+
+    @After
+    public void removeNonDefaultProjects() throws IOException {
+        if (preserveClusterUponCompletion() == false) {
+            cleanUpProjects();
+        }
+    }
+
+    /**
+     * Test <code>POST /_reindex/{task_id}/_cancel</code> API is project-aware, by doing the following:
+     * 1. Create two projects: one will contain a reindex task
+     * 2. Try to cancel the reindex from the other project and expect a 404
+     * 3. Cancel the reindex from the correct project and expect success
+     */
+    public void testCancellingReindexOnlyWorksForCorrectProject() throws Exception {
+        final String projectWithReindex = randomUniqueProjectId().id();
+        final String projectWithoutReindex = randomUniqueProjectId().id();
+
+        createProject(projectWithReindex);
+        createProject(projectWithoutReindex);
+        createPopulatedIndexInProject(SOURCE_INDEX, projectWithReindex);
+
+        final TaskId taskId = startAsyncThrottledReindexInProject(projectWithReindex);
+
+        assertTrue(runningTaskExistsInProject(taskId, projectWithReindex));
+
+        final var cancellingFromOtherProjectException = expectThrows(
+            ResponseException.class,
+            () -> cancelReindexInProjectAndWaitForCompletion(taskId, projectWithoutReindex)
+        );
+        assertThat(cancellingFromOtherProjectException.getResponse().getStatusLine().getStatusCode(), equalTo(404));
+        final String reason = ObjectPath.createFromResponse(cancellingFromOtherProjectException.getResponse()).evaluate("error.reason");
+        assertThat(reason, equalTo(Strings.format("reindex task [%s] either not found or completed", taskId)));
+
+        assertTrue(runningTaskExistsInProject(taskId, projectWithReindex));
+
+        final Map<String, Object> response = cancelReindexInProjectAndWaitForCompletion(taskId, projectWithReindex);
+        assertThat("reindex is cancelled", response, equalTo(Map.of("acknowledged", true)));
+
+        assertFalse(runningTaskExistsInProject(taskId, projectWithReindex));
+    }
+
+    private static TaskId startAsyncThrottledReindexInProject(final String projectId) throws IOException {
+        final Request request = new Request("POST", "/_reindex");
+        setRequestProjectId(request, projectId);
+        request.addParameter("wait_for_completion", "false");
+        request.addParameter("slices", Integer.toString(NUM_OF_SLICES));
+        request.addParameter("requests_per_second", Integer.toString(REQUESTS_PER_SECOND));
+        request.setJsonEntity(Strings.format("""
+            {
+              "source": {
+                "index": "%s",
+                "size": %d
+              },
+              "dest": {
+                "index": "%s"
+              }
+            }
+            """, SOURCE_INDEX, BULK_SIZE, DEST_INDEX));
+
+        final Response response = assertOK(client().performRequest(request));
+        final String task = (String) entityAsMap(response).get("task");
+        assertNotNull("reindex did not return a task id", task);
+        return new TaskId(task);
+    }
+
+    private static Map<String, Object> cancelReindexInProjectAndWaitForCompletion(final TaskId taskId, final String projectId)
+        throws IOException {
+        final Request request = new Request("POST", "/_reindex/" + taskId + "/_cancel");
+        request.addParameter("wait_for_completion", "true");
+        setRequestProjectId(request, projectId);
+        final Response response = assertOK(client().performRequest(request));
+        return entityAsMap(response);
+    }
+
+    private static boolean runningTaskExistsInProject(final TaskId taskId, final String projectId) throws IOException {
+        final Request request = new Request("GET", "/_tasks/" + taskId);
+        setRequestProjectId(request, projectId);
+        try {
+            final Response response = assertOK(client().performRequest(request));
+            final Map<String, Object> body = entityAsMap(response);
+            return Boolean.FALSE.equals(body.get("completed"));
+        } catch (ResponseException e) {
+            if (e.getResponse().getStatusLine().getStatusCode() == 404) {
+                return false;
+            }
+            throw e;
+        }
+    }
+
+    private static void createPopulatedIndexInProject(final String indexName, final String projectId) throws IOException {
+        createIndex(request -> {
+            setRequestProjectId(request, projectId);
+            return assertOK(client().performRequest(request));
+        }, indexName, null, null, null);
+
+        final Request bulkRequest = new Request("POST", "/_bulk");
+        setRequestProjectId(bulkRequest, projectId);
+        bulkRequest.addParameter("refresh", "true");
+
+        final StringBuilder bulkBody = new StringBuilder();
+        for (int i = 0; i < NUMBER_OF_DOCUMENTS_THAT_TAKES_30_SECS_TO_INGEST; i++) {
+            bulkBody.append(Strings.format("""
+                {"index":{"_index":"%s"}}
+                {"value": %d}
+                """, indexName, i));
+        }
+        bulkRequest.setJsonEntity(bulkBody.toString());
+
+        final Response bulkResponse = assertOK(client().performRequest(bulkRequest));
+        final Map<String, Object> bulkResult = entityAsMap(bulkResponse);
+        assertThat("bulk index didn't receive errors", bulkResult.get("errors"), equalTo(false));
+    }
+
+    private static void setRequestProjectId(final Request request, final String projectId) {
+        final RequestOptions.Builder options = request.getOptions().toBuilder();
+        options.removeHeader(Task.X_ELASTIC_PROJECT_ID_HTTP_HEADER);
+        options.addHeader(Task.X_ELASTIC_PROJECT_ID_HTTP_HEADER, projectId);
+        request.setOptions(options);
+    }
+}