diff --git a/x-pack/solutions/observability/plugins/synthetics/server/routes/filters/filters.ts b/x-pack/solutions/observability/plugins/synthetics/server/routes/filters/filters.ts
index aab30d16226df..cc2a4cc758cc8 100644
--- a/x-pack/solutions/observability/plugins/synthetics/server/routes/filters/filters.ts
+++ b/x-pack/solutions/observability/plugins/synthetics/server/routes/filters/filters.ts
@@ -10,6 +10,7 @@ import {
   legacySyntheticsMonitorTypeSingle,
   syntheticsMonitorAttributes,
   syntheticsMonitorSavedObjectType,
+  legacyMonitorAttributes,
 } from '../../../common/types/saved_objects';
 import { ConfigKey, MonitorFiltersResult } from '../../../common/runtime_types';
 import { SYNTHETICS_API_URLS } from '../../../common/constants';
@@ -20,23 +21,29 @@ type Buckets = Array<{
 }>;
 
 interface AggsResponse {
-  monitorTypes: {
-    buckets: Buckets;
-  };
-  locations: {
-    buckets: Buckets;
-  };
-  tags: {
-    buckets: Buckets;
-  };
-  projects: {
-    buckets: Buckets;
-  };
-  schedules: {
-    buckets: Buckets;
-  };
+  monitorTypes?: { buckets: Buckets };
+  monitorTypesLegacy?: { buckets: Buckets };
+  locations?: { buckets: Buckets };
+  locationsLegacy?: { buckets: Buckets };
+  tags?: { buckets: Buckets };
+  tagsLegacy?: { buckets: Buckets };
+  projects?: { buckets: Buckets };
+  projectsLegacy?: { buckets: Buckets };
+  schedules?: { buckets: Buckets };
+  schedulesLegacy?: { buckets: Buckets };
 }
 
+const mergeBuckets = (...bucketSets: Array<Buckets | undefined>) => {
+  const map = new Map<string, number>();
+  for (const buckets of bucketSets) {
+    buckets?.forEach(({ key, doc_count: docCount }) => {
+      const k = String(key);
+      map.set(k, (map.get(k) ?? 0) + docCount);
+    });
+  }
+  return Array.from(map.entries()).map(([label, count]) => ({ label, count }));
+};
+
 export const getSyntheticsFilters: SyntheticsRestApiRouteFactory<MonitorFiltersResult> = () => ({
   method: 'GET',
   path: SYNTHETICS_API_URLS.FILTERS,
@@ -54,36 +61,29 @@ export const getSyntheticsFilters: SyntheticsRestApiRouteFactory<MonitorFiltersR
       ...(showFromAllSpaces ? { namespaces: ['*'] } : {}),
     });
 
-    const { monitorTypes, tags, locations, projects, schedules } =
-      (data?.aggregations as AggsResponse) ?? {};
+    const {
+      monitorTypes,
+      monitorTypesLegacy,
+      tags,
+      tagsLegacy,
+      locations,
+      locationsLegacy,
+      projects,
+      projectsLegacy,
+      schedules,
+      schedulesLegacy,
+    } = (data?.aggregations as AggsResponse) ?? {};
+
     return {
-      monitorTypes:
-        monitorTypes?.buckets?.map(({ key, doc_count: count }) => ({
-          label: key,
-          count,
-        })) ?? [],
-      tags:
-        tags?.buckets?.map(({ key, doc_count: count }) => ({
-          label: key,
-          count,
-        })) ?? [],
-      locations:
-        locations?.buckets?.map(({ key, doc_count: count }) => ({
-          label: key,
-          count,
-        })) ?? [],
-      projects:
-        projects?.buckets
-          ?.filter(({ key }) => key)
-          .map(({ key, doc_count: count }) => ({
-            label: key,
-            count,
-          })) ?? [],
-      schedules:
-        schedules?.buckets?.map(({ key, doc_count: count }) => ({
-          label: String(key),
-          count,
-        })) ?? [],
+      monitorTypes: mergeBuckets(monitorTypes?.buckets, monitorTypesLegacy?.buckets),
+      tags: mergeBuckets(tags?.buckets, tagsLegacy?.buckets),
+      locations: mergeBuckets(locations?.buckets, locationsLegacy?.buckets),
+      projects: mergeBuckets(projects?.buckets, projectsLegacy?.buckets).filter(
+        ({ label }) => label
+      ),
+      schedules: mergeBuckets(schedules?.buckets, schedulesLegacy?.buckets).map(
+        ({ label, count }) => ({ label: String(label), count })
+      ),
     };
   },
 });
@@ -95,28 +95,58 @@ const aggs = {
       size: 10000,
     },
   },
+  monitorTypesLegacy: {
+    terms: {
+      field: `${legacyMonitorAttributes}.${ConfigKey.MONITOR_TYPE}.keyword`,
+      size: 10000,
+    },
+  },
   tags: {
     terms: {
       field: `${syntheticsMonitorAttributes}.${ConfigKey.TAGS}`,
       size: 10000,
     },
   },
+  tagsLegacy: {
+    terms: {
+      field: `${legacyMonitorAttributes}.${ConfigKey.TAGS}`,
+      size: 10000,
+    },
+  },
   locations: {
     terms: {
       field: `${syntheticsMonitorAttributes}.${ConfigKey.LOCATIONS}.id`,
       size: 10000,
     },
   },
+  locationsLegacy: {
+    terms: {
+      field: `${legacyMonitorAttributes}.${ConfigKey.LOCATIONS}.id`,
+      size: 10000,
+    },
+  },
   projects: {
     terms: {
       field: `${syntheticsMonitorAttributes}.${ConfigKey.PROJECT_ID}`,
       size: 10000,
     },
   },
+  projectsLegacy: {
+    terms: {
+      field: `${legacyMonitorAttributes}.${ConfigKey.PROJECT_ID}`,
+      size: 10000,
+    },
+  },
   schedules: {
     terms: {
       field: `${syntheticsMonitorAttributes}.${ConfigKey.SCHEDULE}.number`,
       size: 10000,
     },
   },
+  schedulesLegacy: {
+    terms: {
+      field: `${legacyMonitorAttributes}.${ConfigKey.SCHEDULE}.number`,
+      size: 10000,
+    },
+  },
 };
diff --git a/x-pack/solutions/observability/test/api_integration_deployment_agnostic/apis/synthetics/get_filters.ts b/x-pack/solutions/observability/test/api_integration_deployment_agnostic/apis/synthetics/get_filters.ts
index 6c028eed8cf20..ab14b92adbb75 100644
--- a/x-pack/solutions/observability/test/api_integration_deployment_agnostic/apis/synthetics/get_filters.ts
+++ b/x-pack/solutions/observability/test/api_integration_deployment_agnostic/apis/synthetics/get_filters.ts
@@ -8,10 +8,14 @@
 import { SYNTHETICS_API_URLS } from '@kbn/synthetics-plugin/common/constants';
 import { RoleCredentials } from '@kbn/ftr-common-functional-services';
 import expect from '@kbn/expect';
-import { PrivateLocation } from '@kbn/synthetics-plugin/common/runtime_types';
-import { syntheticsMonitorSavedObjectType } from '@kbn/synthetics-plugin/common/types/saved_objects';
-import { DeploymentAgnosticFtrProviderContext } from '../../ftr_provider_context';
+import type { PrivateLocation } from '@kbn/synthetics-plugin/common/runtime_types';
+import {
+  legacySyntheticsMonitorTypeSingle,
+  syntheticsMonitorSavedObjectType,
+} from '@kbn/synthetics-plugin/common/types/saved_objects';
+import type { DeploymentAgnosticFtrProviderContext } from '../../ftr_provider_context';
 import { PrivateLocationTestService } from '../../services/synthetics_private_location';
+import { addMonitorAPIHelper } from './create_monitor';
 
 export default function ({ getService }: DeploymentAgnosticFtrProviderContext) {
   describe('getMonitorFilters', function () {
@@ -25,15 +29,23 @@ export default function ({ getService }: DeploymentAgnosticFtrProviderContext) {
     let privateLocation: PrivateLocation;
 
     after(async () => {
-      await kibanaServer.savedObjects.clean({ types: [syntheticsMonitorSavedObjectType] });
+      await kibanaServer.savedObjects.clean({
+        types: [syntheticsMonitorSavedObjectType, legacySyntheticsMonitorTypeSingle],
+      });
     });
 
     before(async () => {
-      await kibanaServer.savedObjects.clean({ types: [syntheticsMonitorSavedObjectType] });
+      await kibanaServer.savedObjects.clean({
+        types: [syntheticsMonitorSavedObjectType, legacySyntheticsMonitorTypeSingle],
+      });
       editorUser = await samlAuth.createM2mApiKeyWithRoleScope('editor');
       privateLocation = await privateLocationTestService.addTestPrivateLocation();
     });
 
+    const addMonitor = async (monitor: any, type?: string) => {
+      return addMonitorAPIHelper(supertest, monitor, 200, editorUser, samlAuth, false, type);
+    };
+
     it('get list of filters', async () => {
       const apiResponse = await supertest
         .get(SYNTHETICS_API_URLS.FILTERS)
@@ -59,12 +71,7 @@ export default function ({ getService }: DeploymentAgnosticFtrProviderContext) {
         locations: [privateLocation],
       };
 
-      await supertest
-        .post(SYNTHETICS_API_URLS.SYNTHETICS_MONITORS)
-        .set(editorUser.apiKeyHeader)
-        .set(samlAuth.getInternalRequestHeader())
-        .send(newMonitor)
-        .expect(200);
+      await addMonitor(newMonitor);
 
       const apiResponse = await supertest
         .get(SYNTHETICS_API_URLS.FILTERS)
@@ -83,5 +90,83 @@ export default function ({ getService }: DeploymentAgnosticFtrProviderContext) {
         schedules: [{ label: '3', count: 1 }],
       });
     });
+
+    it('get list of filters for legacy saved object type monitor', async () => {
+      // Create a legacy monitor directly via savedObjectsClient
+
+      // Use the internal savedObjectsClient to create a legacy monitor
+      await addMonitor(
+        {
+          name: 'Legacy Monitor',
+          type: 'icmp',
+          host: 'https://legacy.elastic.co',
+          tags: ['legacy', 'synthetics'],
+          locations: [privateLocation],
+        },
+        legacySyntheticsMonitorTypeSingle
+      );
+
+      const apiResponse = await supertest
+        .get(SYNTHETICS_API_URLS.FILTERS)
+        .set(editorUser.apiKeyHeader)
+        .set(samlAuth.getInternalRequestHeader())
+        .expect(200);
+
+      expect(apiResponse.body.monitorTypes).to.eql([
+        { label: 'http', count: 1 },
+        { label: 'icmp', count: 1 },
+      ]);
+      expect(apiResponse.body.tags).to.eql([
+        { label: 'apm', count: 1 },
+        { label: 'synthetics', count: 2 },
+        { label: 'legacy', count: 1 },
+      ]);
+      expect(apiResponse.body.locations).to.eql([{ label: privateLocation.id, count: 2 }]);
+      expect(apiResponse.body.schedules).to.eql([{ label: '3', count: 2 }]);
+    });
+
+    it('get list of filters with both legacy and modern monitors', async () => {
+      // Create a modern monitor
+      const modernMonitor = {
+        name: 'Modern Monitor',
+        type: 'http',
+        urls: 'https://modern.elastic.co',
+        tags: ['multi-space', 'synthetics'],
+        locations: [privateLocation],
+      };
+
+      await addMonitor(modernMonitor);
+
+      await addMonitor(
+        {
+          name: 'Legacy Monitor 3',
+          type: 'icmp',
+          host: 'https://legacy2.elastic.co',
+          tags: ['legacy2', 'synthetics'],
+          locations: [privateLocation],
+        },
+        legacySyntheticsMonitorTypeSingle
+      );
+
+      const apiResponse = await supertest
+        .get(SYNTHETICS_API_URLS.FILTERS)
+        .set(editorUser.apiKeyHeader)
+        .set(samlAuth.getInternalRequestHeader())
+        .expect(200);
+
+      expect(apiResponse.body.monitorTypes).to.eql([
+        { label: 'http', count: 2 },
+        { label: 'icmp', count: 2 },
+      ]);
+      expect(apiResponse.body.tags).to.eql([
+        { label: 'synthetics', count: 4 },
+        { label: 'apm', count: 1 },
+        { label: 'multi-space', count: 1 },
+        { label: 'legacy', count: 1 },
+        { label: 'legacy2', count: 1 },
+      ]);
+      expect(apiResponse.body.locations).to.eql([{ label: privateLocation.id, count: 4 }]);
+      expect(apiResponse.body.schedules).to.eql([{ label: '3', count: 4 }]);
+    });
   });
 }