diff --git a/plugins/scaffolder-backend/src/scaffolder/tasks/DefaultWorkflowRunner.test.ts b/plugins/scaffolder-backend/src/scaffolder/tasks/DefaultWorkflowRunner.test.ts
index c249606658..6447782654 100644
--- a/plugins/scaffolder-backend/src/scaffolder/tasks/DefaultWorkflowRunner.test.ts
+++ b/plugins/scaffolder-backend/src/scaffolder/tasks/DefaultWorkflowRunner.test.ts
@@ -55,6 +55,23 @@ describe('DefaultWorkflowRunner', () => {
       handler: fakeActionHandler,
     });
 
+    actionRegistry.register({
+      id: 'jest-validated-action',
+      description: 'Mock action for testing',
+      handler: fakeActionHandler,
+      schema: {
+        input: {
+          type: 'object',
+          required: ['foo'],
+          properties: {
+            foo: {
+              type: 'number',
+            },
+          },
+        },
+      },
+    });
+
     actionRegistry.register({
       id: 'output-action',
       description: 'Mock action for testing',
@@ -85,7 +102,40 @@ describe('DefaultWorkflowRunner', () => {
     );
   });
 
-  describe('validation', () => {});
+  describe('validation', () => {
+    it('should throw an error if the action has a schema and the input does not match', async () => {
+      const task = createMockTaskWithSpec({
+        apiVersion: 'backstage.io/v1beta3',
+        parameters: {},
+        output: {},
+        steps: [{ id: 'test', name: 'name', action: 'jest-validated-action' }],
+      });
+
+      await expect(runner.execute(task)).rejects.toThrowError(
+        /Invalid input passed to action jest-validated-action, instance requires property \"foo\"/,
+      );
+    });
+
+    it('should run the action when the validation passes', async () => {
+      const task = createMockTaskWithSpec({
+        apiVersion: 'backstage.io/v1beta3',
+        parameters: {},
+        output: {},
+        steps: [
+          {
+            id: 'test',
+            name: 'name',
+            action: 'jest-validated-action',
+            input: { foo: 1 },
+          },
+        ],
+      });
+
+      await runner.execute(task);
+
+      expect(fakeActionHandler).toHaveBeenCalledTimes(1);
+    });
+  });
   describe('conditionals', () => {
     it('should execute steps conditionally', async () => {
       const task = createMockTaskWithSpec({
diff --git a/plugins/scaffolder-backend/src/scaffolder/tasks/DefaultWorkflowRunner.ts b/plugins/scaffolder-backend/src/scaffolder/tasks/DefaultWorkflowRunner.ts
index 889048d04a..c08363857d 100644
--- a/plugins/scaffolder-backend/src/scaffolder/tasks/DefaultWorkflowRunner.ts
+++ b/plugins/scaffolder-backend/src/scaffolder/tasks/DefaultWorkflowRunner.ts
@@ -31,6 +31,7 @@ import { JsonObject, JsonValue } from '@backstage/config';
 import { InputError } from '@backstage/errors';
 import { PassThrough } from 'stream';
 import { isTruthy } from './helper';
+import { validate as validateJsonSchema } from 'jsonschema';
 
 type Options = {
   workingDirectory: string;
@@ -79,6 +80,8 @@ export class DefaultWorkflowRunner implements WorkflowRunner {
   private readonly nunjucks: nunjucks.Environment;
 
   constructor(private readonly options: Options) {
+    // TODO(blam): Probably need the repo helper here.
+    // Or we move to returning Objects in the RepoUrlPickerV2 or something?
     this.nunjucks = nunjucks.configure({
       autoescape: false,
       tags: {
@@ -164,7 +167,20 @@ export class DefaultWorkflowRunner implements WorkflowRunner {
           const action = this.options.actionRegistry.get(step.action);
           const { taskLogger, streamLogger } = createStepLogger({ task, step });
 
-          const input = step.input && this.render(step.input, context);
+          const input = (step.input && this.render(step.input, context)) ?? {};
+
+          if (action.schema?.input) {
+            const validateResult = validateJsonSchema(
+              input,
+              action.schema.input,
+            );
+            if (!validateResult.valid) {
+              const errors = validateResult.errors.join(', ');
+              throw new InputError(
+                `Invalid input passed to action ${action.id}, ${errors}`,
+              );
+            }
+          }
 
           const tmpDirs = new Array<string>();
           const stepOutput: { [outputName: string]: JsonValue } = {};