diff --git a/code/lib/cli-storybook/src/codemod/helpers/config-to-csf-factory.test.ts b/code/lib/cli-storybook/src/codemod/helpers/config-to-csf-factory.test.ts
index eb20715486f5..8c22d4fbba96 100644
--- a/code/lib/cli-storybook/src/codemod/helpers/config-to-csf-factory.test.ts
+++ b/code/lib/cli-storybook/src/codemod/helpers/config-to-csf-factory.test.ts
@@ -347,7 +347,7 @@ describe('preview specific functionality', () => {
         import { type Preview } from '@storybook/react-vite';
         export const decorators = []
         const preview = {
-          
+
           parameters: {
             options: {}
           }
@@ -368,4 +368,42 @@ describe('preview specific functionality', () => {
       });
     `);
   });
+
+  it('should add default export when preview only has side-effect imports', async () => {
+    await expect(
+      transform(dedent`
+        import './preview.scss'
+      `)
+    ).resolves.toMatchInlineSnapshot(`
+      import { definePreview } from '@storybook/react-vite';
+
+      import './preview.scss';
+
+      export default definePreview({});
+    `);
+  });
+
+  it('should add default export when preview file is empty', async () => {
+    await expect(transform('')).resolves.toMatchInlineSnapshot(`
+      import { definePreview } from '@storybook/react-vite';
+
+      export default definePreview({});
+    `);
+  });
+
+  it('should add default export when preview only has multiple side-effect imports', async () => {
+    await expect(
+      transform(dedent`
+        import './preview.scss'
+        import './global.css'
+      `)
+    ).resolves.toMatchInlineSnapshot(`
+      import { definePreview } from '@storybook/react-vite';
+
+      import './global.css';
+      import './preview.scss';
+
+      export default definePreview({});
+    `);
+  });
 });
diff --git a/code/lib/cli-storybook/src/codemod/helpers/config-to-csf-factory.ts b/code/lib/cli-storybook/src/codemod/helpers/config-to-csf-factory.ts
index 631e7677abfb..c289c247ff51 100644
--- a/code/lib/cli-storybook/src/codemod/helpers/config-to-csf-factory.ts
+++ b/code/lib/cli-storybook/src/codemod/helpers/config-to-csf-factory.ts
@@ -166,6 +166,21 @@ export async function configToCsfFactory(
 
     // Add the new export default declaration
     programNode.body.push(t.exportDefaultDeclaration(defineConfigCall));
+  } else if (configType === 'preview') {
+    /**
+     * Scenario 4: No exports (empty file or only side-effect imports)
+     *
+     * ```
+     * import './preview.scss';
+     * ```
+     *
+     * Transform into: `import './preview.scss'; export default definePreview({})`
+     *
+     * This is needed because story files using CSF factories import from preview, so the preview
+     * file must have a default export.
+     */
+    const defineConfigCall = t.callExpression(t.identifier(methodName), [t.objectExpression([])]);
+    programNode.body.push(t.exportDefaultDeclaration(defineConfigCall));
   }
 
   const configImport = t.importDeclaration(