diff --git a/packages/booter-lb3app/src/__tests__/acceptance/booter-lb3app.acceptance.ts b/packages/booter-lb3app/src/__tests__/acceptance/booter-lb3app.acceptance.ts
index a9fbc36ed8e0..6563ba4fbb99 100644
--- a/packages/booter-lb3app/src/__tests__/acceptance/booter-lb3app.acceptance.ts
+++ b/packages/booter-lb3app/src/__tests__/acceptance/booter-lb3app.acceptance.ts
@@ -3,7 +3,7 @@
 // This file is licensed under the MIT License.
 // License text available at https://opensource.org/licenses/MIT
 
-import {OperationObject} from '@loopback/rest';
+import {OperationObject, OpenApiSpec} from '@loopback/rest';
 import {Client, expect} from '@loopback/testlab';
 import * as _ from 'lodash';
 import {
@@ -189,4 +189,30 @@ describe('booter-lb3app', () => {
       await client.get('/coffee').expect(404);
     });
   });
+
+  context('using specTransformer to modify OpenAPI spec', () => {
+    before(async () => {
+      ({app, client} = await setupApplication({
+        lb3app: {
+          path: '../fixtures/lb3app/server/server',
+          specTransformer: (spec: OpenApiSpec): OpenApiSpec =>
+            _.merge(spec, {
+              paths: {
+                '/CoffeeShops': {
+                  post: {
+                    summary: 'just a very simple modification',
+                  },
+                },
+              },
+            }),
+        },
+      }));
+    });
+
+    it('does apply the spec modification', () => {
+      const spec = app.restServer.getApiSpec();
+      const createOp: OperationObject = spec.paths['/api/CoffeeShops'].post;
+      expect(createOp.summary).to.eql('just a very simple modification');
+    });
+  });
 });
diff --git a/packages/booter-lb3app/src/lb3app.booter.ts b/packages/booter-lb3app/src/lb3app.booter.ts
index ed354f2c8fa5..2d6b1c644726 100644
--- a/packages/booter-lb3app/src/lb3app.booter.ts
+++ b/packages/booter-lb3app/src/lb3app.booter.ts
@@ -101,7 +101,11 @@ export class Lb3AppBooter implements Booter {
       // swagger2openapi options
     });
 
-    return result.openapi as OpenApiSpec;
+    let spec = result.openapi as OpenApiSpec;
+    if (typeof this.options.specTransformer === 'function') {
+      spec = this.options.specTransformer(spec);
+    }
+    return spec;
   }
 
   private mountFullApp(lb3App: Lb3Application, spec: OpenApiSpec) {
@@ -128,6 +132,7 @@ export interface Lb3AppBooterOptions {
   path: string;
   mode: 'fullApp' | 'restRouter';
   restApiRoot: string;
+  specTransformer?: (spec: OpenApiSpec) => OpenApiSpec;
 }
 
 interface Lb3Application extends ExpressApplication {