diff --git a/x-pack/test/functional/es_archives/signals/legacy_signals_index_non_default_space/data.json b/x-pack/test/functional/es_archives/signals/legacy_signals_index_non_default_space/data.json
new file mode 100644
index 0000000000000..2547b46171e44
--- /dev/null
+++ b/x-pack/test/functional/es_archives/signals/legacy_signals_index_non_default_space/data.json
@@ -0,0 +1,12 @@
+{
+    "type": "doc",
+    "value": {
+      "id": "1",
+      "index": ".siem-signals-another-space-legacy",
+      "source": {
+        "@timestamp": "2020-10-10T00:00:00.000Z",
+        "signal": {}
+      },
+      "type": "_doc"
+    }
+  }
\ No newline at end of file
diff --git a/x-pack/test/functional/es_archives/signals/legacy_signals_index_non_default_space/mappings.json b/x-pack/test/functional/es_archives/signals/legacy_signals_index_non_default_space/mappings.json
new file mode 100644
index 0000000000000..0dbbf1d4e833e
--- /dev/null
+++ b/x-pack/test/functional/es_archives/signals/legacy_signals_index_non_default_space/mappings.json
@@ -0,0 +1,29 @@
+{
+    "type": "index",
+    "value": {
+      "aliases": {
+        ".siem-signals-another-space": {
+          "is_write_index": false
+        }
+      },
+      "index": ".siem-signals-another-space-legacy",
+      "mappings": {
+        "_meta": {
+          "version": 1
+        },
+        "properties": {
+          "@timestamp": {
+            "type": "date"
+          },
+          "signal": { "type": "object" }
+        }
+      },
+      "settings": {
+        "index": {
+          "lifecycle": {
+            "indexing_complete": true
+          }
+        }
+      }
+    }
+  }
\ No newline at end of file
diff --git a/x-pack/test/security_solution_api_integration/test_suites/detections_response/detection_engine/alerts/basic_license_essentials_tier/ess_specific_index_logic/migrations/deprecations.ts b/x-pack/test/security_solution_api_integration/test_suites/detections_response/detection_engine/alerts/basic_license_essentials_tier/ess_specific_index_logic/migrations/deprecations.ts
new file mode 100644
index 0000000000000..77dcede0c0491
--- /dev/null
+++ b/x-pack/test/security_solution_api_integration/test_suites/detections_response/detection_engine/alerts/basic_license_essentials_tier/ess_specific_index_logic/migrations/deprecations.ts
@@ -0,0 +1,94 @@
+/*
+ * 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; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+import expect from 'expect';
+import type { DeprecationsDetails } from '@kbn/core/server';
+
+import {
+  createAlertsIndex,
+  deleteAllAlerts,
+} from '../../../../../../../../common/utils/security_solution';
+
+import { FtrProviderContext } from '../../../../../../../ftr_provider_context';
+
+export default ({ getService }: FtrProviderContext): void => {
+  const esArchiver = getService('esArchiver');
+  const supertest = getService('supertest');
+  const log = getService('log');
+  const es = getService('es');
+
+  const getDeprecations = async (): Promise<DeprecationsDetails[]> => {
+    const { body } = await supertest.get('/api/deprecations/').set('kbn-xsrf', 'true').expect(200);
+    return body.deprecations;
+  };
+
+  const getLegacyIndicesDeprecation = async (): Promise<DeprecationsDetails | undefined> => {
+    const deprecations = await getDeprecations();
+
+    return deprecations.find(({ title }) => title === 'Found not migrated detection alerts');
+  };
+
+  describe.only('@ess Alerts migration deprecations API', () => {
+    describe('no siem legacy indices exist', () => {
+      it('should return empty siem signals deprecation', async () => {
+        const deprecation = await getLegacyIndicesDeprecation();
+
+        expect(deprecation).toBeUndefined();
+      });
+    });
+
+    describe('siem legacy indices exist', () => {
+      beforeEach(async () => {
+        await esArchiver.load('x-pack/test/functional/es_archives/signals/legacy_signals_index');
+        await createAlertsIndex(supertest, log);
+      });
+
+      afterEach(async () => {
+        await esArchiver.unload('x-pack/test/functional/es_archives/signals/legacy_signals_index');
+        await deleteAllAlerts(supertest, log, es);
+      });
+
+      it('should return legacy siem signals deprecation', async () => {
+        const deprecation = await getLegacyIndicesDeprecation();
+
+        expect(deprecation?.level).toBe('warning');
+
+        // ensures space included in manual steps
+        expect(deprecation?.correctiveActions.manualSteps[1]).toContain(
+          'Spaces with at least one non-migrated signals index: default.'
+        );
+        expect(deprecation?.correctiveActions.manualSteps[2]).toContain(
+          'Example of migration API calls:'
+        );
+        expect(deprecation?.correctiveActions.manualSteps[3]).toContain(
+          'GET <kibana host>:<port>/api/detection_engine/signals/migration_status?from=1970-01-01T00:00:00.000Z'
+        );
+      });
+
+      describe('multiple spaces', () => {
+        beforeEach(async () => {
+          await esArchiver.load(
+            'x-pack/test/functional/es_archives/signals/legacy_signals_index_non_default_space'
+          );
+        });
+
+        afterEach(async () => {
+          await esArchiver.unload(
+            'x-pack/test/functional/es_archives/signals/legacy_signals_index_non_default_space'
+          );
+        });
+
+        it('should return legacy siem signals deprecation with multiple spaces', async () => {
+          const deprecation = await getLegacyIndicesDeprecation();
+
+          expect(deprecation?.correctiveActions.manualSteps[1]).toContain('another-space');
+          expect(deprecation?.correctiveActions.manualSteps[1]).toContain('default');
+        });
+      });
+    });
+  });
+};
diff --git a/x-pack/test/security_solution_api_integration/test_suites/detections_response/detection_engine/alerts/basic_license_essentials_tier/ess_specific_index_logic/migrations/index.ts b/x-pack/test/security_solution_api_integration/test_suites/detections_response/detection_engine/alerts/basic_license_essentials_tier/ess_specific_index_logic/migrations/index.ts
index 2c1aed2b1387b..2266e653b2493 100644
--- a/x-pack/test/security_solution_api_integration/test_suites/detections_response/detection_engine/alerts/basic_license_essentials_tier/ess_specific_index_logic/migrations/index.ts
+++ b/x-pack/test/security_solution_api_integration/test_suites/detections_response/detection_engine/alerts/basic_license_essentials_tier/ess_specific_index_logic/migrations/index.ts
@@ -12,5 +12,6 @@ export default function ({ loadTestFile }: FtrProviderContext) {
     loadTestFile(require.resolve('./delete_alerts_migrations'));
     loadTestFile(require.resolve('./finalize_alerts_migrations'));
     loadTestFile(require.resolve('./get_alerts_migration_status'));
+    loadTestFile(require.resolve('./deprecations'));
   });
 }