diff --git a/src/lib/routes/admin-api/project/environments.ts b/src/lib/routes/admin-api/project/environments.ts
index 0e9bcd5cd9..30541c246a 100644
--- a/src/lib/routes/admin-api/project/environments.ts
+++ b/src/lib/routes/admin-api/project/environments.ts
@@ -17,7 +17,7 @@ import {
     getStandardResponses,
     ProjectEnvironmentSchema,
 } from '../../../openapi';
-import { OpenApiService } from '../../../services';
+import { OpenApiService, ProjectService } from '../../../services';
 
 const PREFIX = '/:projectId/environments';
 
@@ -33,18 +33,25 @@ export default class EnvironmentsController extends Controller {
 
     private openApiService: OpenApiService;
 
+    private projectService: ProjectService;
+
     constructor(
         config: IUnleashConfig,
         {
             environmentService,
             openApiService,
-        }: Pick<IUnleashServices, 'environmentService' | 'openApiService'>,
+            projectService,
+        }: Pick<
+            IUnleashServices,
+            'environmentService' | 'openApiService' | 'projectService'
+        >,
     ) {
         super(config);
 
         this.logger = config.getLogger('admin-api/project/environments.ts');
         this.environmentService = environmentService;
         this.openApiService = openApiService;
+        this.projectService = projectService;
 
         this.route({
             method: 'post',
@@ -126,6 +133,7 @@ export default class EnvironmentsController extends Controller {
     ): Promise<void> {
         const { projectId } = req.params;
         const { environment } = req.body;
+        await this.projectService.getProject(projectId); // Validates that the project exists
 
         await this.environmentService.addEnvironmentToProject(
             environment,
diff --git a/src/test/e2e/api/admin/project/environments.e2e.test.ts b/src/test/e2e/api/admin/project/environments.e2e.test.ts
index 7d50aca4ca..d951841aa0 100644
--- a/src/test/e2e/api/admin/project/environments.e2e.test.ts
+++ b/src/test/e2e/api/admin/project/environments.e2e.test.ts
@@ -159,3 +159,12 @@ test('Should throw an error if you try to set defaultStrategy other than flexibl
         })
         .expect(400);
 });
+
+test('Add environment to project should return 404 when given a projectid that does not exist', async () => {
+    await app.request
+        .post(`/api/admin/projects/unknown/environments`)
+        .send({
+            environment: 'default',
+        })
+        .expect(404);
+});