diff --git a/lib/runner_base.js b/lib/runner_base.js
index 85e7cc6..663635a 100644
--- a/lib/runner_base.js
+++ b/lib/runner_base.js
@@ -78,9 +78,12 @@ class RunnerBase {
     if (configFilePath) {
       await this.loadSpecificConfigFile_(configFilePath);
     } else {
+      let numFound = 0;
+
       for (const ext of ['json', 'js']) {
         try {
           await this.loadSpecificConfigFile_(`spec/support/jasmine.${ext}`);
+          numFound++;
         } catch (e) {
           if (e.code !== 'MODULE_NOT_FOUND'       // CommonJS
             && e.code !== 'ERR_MODULE_NOT_FOUND'  // ESM
@@ -89,6 +92,15 @@ class RunnerBase {
           }
         }
       }
+
+      if (numFound > 1) {
+        console.warn(
+          'Deprecation warning: Jasmine found and loaded both jasmine.js ' +
+          'and jasmine.json\n' +
+          'config files. In a future version, only the first file found ' +
+          'will be loaded.'
+        );
+      }
     }
   }
 
diff --git a/spec/shared_runner_behaviors.js b/spec/shared_runner_behaviors.js
index 569f9b6..ac4be58 100644
--- a/spec/shared_runner_behaviors.js
+++ b/spec/shared_runner_behaviors.js
@@ -308,6 +308,27 @@ function sharedRunnerBehaviors(makeRunner) {
             pathEndingWith('spec/fixtures/sample_project/spec/fixture_spec.js')
           ]);
         });
+
+        it('warns if both default config files are found', async function() {
+          spyOn(Loader.prototype, 'load').and.callFake(function (path) {
+            if (path.endsWith('jasmine.js') || path.endsWith('jasmine.json')) {
+              return Promise.resolve({});
+            } else {
+              const e = new Error(`Module not found: ${path}`);
+              e.code = 'MODULE_NOT_FOUND';
+              return Promise.reject(e);
+            }
+          });
+          spyOn(console, 'warn');
+
+          await this.fixtureJasmine.loadConfigFile();
+
+          expect(console.warn).toHaveBeenCalledWith(
+            'Deprecation warning: Jasmine found and loaded both jasmine.js ' +
+            'and jasmine.json\nconfig files. In a future version, only the ' +
+            'first file found will be loaded.'
+          );
+        });
       });
     });