diff --git a/.changeset/weak-emus-confess.md b/.changeset/weak-emus-confess.md
new file mode 100644
index 0000000000..4f2b748173
--- /dev/null
+++ b/.changeset/weak-emus-confess.md
@@ -0,0 +1,5 @@
+---
+'@astrojs/mdx': patch
+---
+
+Fix: Add GFM and Smartypants to MDX by default
diff --git a/packages/integrations/mdx/src/utils.ts b/packages/integrations/mdx/src/utils.ts
index 174cad9c11..d4d13ab05e 100644
--- a/packages/integrations/mdx/src/utils.ts
+++ b/packages/integrations/mdx/src/utils.ts
@@ -127,7 +127,7 @@ export async function getRemarkPlugins(
 		default:
 			remarkPlugins = [
 				...remarkPlugins,
-				...(config.markdown.extendDefaultPlugins ? DEFAULT_REMARK_PLUGINS : []),
+				...(markdownShouldExtendDefaultPlugins(config) ? DEFAULT_REMARK_PLUGINS : []),
 				...ignoreStringPlugins(config.markdown.remarkPlugins ?? []),
 			];
 			break;
@@ -162,7 +162,7 @@ export function getRehypePlugins(
 		default:
 			rehypePlugins = [
 				...rehypePlugins,
-				...(config.markdown.extendDefaultPlugins ? DEFAULT_REHYPE_PLUGINS : []),
+				...(markdownShouldExtendDefaultPlugins(config) ? DEFAULT_REHYPE_PLUGINS : []),
 				...ignoreStringPlugins(config.markdown.rehypePlugins ?? []),
 			];
 			break;
@@ -172,6 +172,13 @@ export function getRehypePlugins(
 	return rehypePlugins;
 }
 
+function markdownShouldExtendDefaultPlugins(config: AstroConfig): boolean {
+	return (
+		config.markdown.extendDefaultPlugins ||
+		(config.markdown.remarkPlugins.length === 0 && config.markdown.rehypePlugins.length === 0)
+	);
+}
+
 function ignoreStringPlugins(plugins: any[]) {
 	let validPlugins: PluggableList = [];
 	let hasInvalidPlugin = false;
diff --git a/packages/integrations/mdx/test/mdx-plugins.test.js b/packages/integrations/mdx/test/mdx-plugins.test.js
index a5c9480ae5..4cbfaae3ef 100644
--- a/packages/integrations/mdx/test/mdx-plugins.test.js
+++ b/packages/integrations/mdx/test/mdx-plugins.test.js
@@ -24,6 +24,17 @@ describe('MDX plugins', () => {
 		expect(selectTocLink(document)).to.not.be.null;
 	});
 
+	it('Applies GFM by default', async () => {
+		const fixture = await buildFixture({
+			integrations: [mdx()],
+		});
+
+		const html = await fixture.readFile(FILE);
+		const { document } = parseHTML(html);
+
+		expect(selectGfmLink(document)).to.not.be.null;
+	});
+
 	it('supports custom rehype plugins', async () => {
 		const fixture = await buildFixture({
 			integrations: [