diff --git a/README.md b/README.md
index e2177a1..b4bee09 100644
--- a/README.md
+++ b/README.md
@@ -5,7 +5,7 @@
 A plugin for [Fastify](https://fastify.dev) to connect routes with a OpenAPI 3.x specification. It does so by:
 
 - Providing a way to register routes using the `operationId` defined in your specification instead of having to manually call `fastify.route` with the correct URL, method, and schema.
-- Handling `securitySchemes` and `security` keywords defined in your OpenAPI specification, simplifying the implementation of authentication and authorization middleware.
+- Handling `securitySchemes` and `security` keywords defined in your specification, simplifying the implementation of authentication and authorization middleware.
 
 ## Installation
 
diff --git a/src/parser/spec.js b/src/parser/spec.js
index 8674007..2f2270b 100644
--- a/src/parser/spec.js
+++ b/src/parser/spec.js
@@ -1,5 +1,14 @@
 import OpenAPIParser from '@readme/openapi-parser';
 
 export const validateSpec = async options => {
-  return await OpenAPIParser.validate(options.spec);
+  const spec = await OpenAPIParser.validate(options.spec);
+
+  const version = spec.openapi ?? spec.swagger;
+  const majorVersion = Number(version?.split?.('.')[0]);
+
+  if (isNaN(majorVersion) || majorVersion < 3 || majorVersion >= 4) {
+    throw new TypeError(`Unsupported OpenAPI version: ${version ?? 'unknown'}`);
+  }
+
+  return spec;
 };
diff --git a/src/parser/spec.test.js b/src/parser/spec.test.js
index 0023c2f..4d16c2c 100644
--- a/src/parser/spec.test.js
+++ b/src/parser/spec.test.js
@@ -9,6 +9,31 @@ describe('validateSpec()', () => {
     );
   });
 
+  it('should throw an error on unsupported OpenAPI versions', async () => {
+    const specSwagger2 = {
+      info: {
+        title: 'Title',
+        version: '1'
+      },
+      paths: {},
+      swagger: '2.0'
+    };
+    const specOpenApi2 = {
+      info: {},
+      openapi: '2.0.0',
+      paths: {}
+    };
+    const specOpenApi4 = {
+      info: {},
+      openapi: '4.0.0',
+      paths: {}
+    };
+
+    await expect(validateSpec({ spec: specSwagger2 })).rejects.toThrowError(/Unsupported OpenAPI version: 2\.0/);
+    await expect(validateSpec({ spec: specOpenApi2 })).rejects.toThrowError(/Unsupported OpenAPI version: 2\.0\.0/);
+    await expect(validateSpec({ spec: specOpenApi4 })).rejects.toThrowError(/Unsupported OpenAPI version: 4\.0\.0/);
+  });
+
   it('should return a parsed spec', async () => {
     const spec = {
       info: {