diff --git a/.changeset/strong-squids-hide.md b/.changeset/strong-squids-hide.md
new file mode 100644
index 000000000000..7d4793ff4d4f
--- /dev/null
+++ b/.changeset/strong-squids-hide.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Set import.meta.env.BASE_URL in dev mode
diff --git a/packages/astro/src/vite-plugin-env/index.ts b/packages/astro/src/vite-plugin-env/index.ts
index 565ea0dae0bb..af9d2d557de5 100644
--- a/packages/astro/src/vite-plugin-env/index.ts
+++ b/packages/astro/src/vite-plugin-env/index.ts
@@ -79,6 +79,7 @@ export default function envVitePlugin({
 				if (privateEnv) {
 					privateEnv.SITE = astroConfig.site ? `'${astroConfig.site}'` : 'undefined';
 					privateEnv.SSR = JSON.stringify(true);
+					privateEnv.BASE_URL = astroConfig.base ? `'${astroConfig.base}'` : undefined;
 					const entries = Object.entries(privateEnv).map(([key, value]) => [
 						`import.meta.env.${key}`,
 						value,
@@ -88,6 +89,7 @@ export default function envVitePlugin({
 					replacements = Object.assign(replacements, {
 						'import.meta.env.SITE': astroConfig.site ? `'${astroConfig.site}'` : 'undefined',
 						'import.meta.env.SSR': JSON.stringify(true),
+						'import.meta.env.BASE_URL': astroConfig.base ? `'${astroConfig.base}'` : undefined,
 						// This catches destructed `import.meta.env` calls,
 						// BUT we only want to inject private keys referenced in the file.
 						// We overwrite this value on a per-file basis.
diff --git a/packages/astro/test/astro-envs.test.js b/packages/astro/test/astro-envs.test.js
index 5c5f410c4b9b..1619d0161708 100644
--- a/packages/astro/test/astro-envs.test.js
+++ b/packages/astro/test/astro-envs.test.js
@@ -1,85 +1,123 @@
 import { expect } from 'chai';
 import { loadFixture } from './test-utils.js';
+import * as cheerio from 'cheerio';
 
 describe('Environment Variables', () => {
+	/** @type {import('./test-utils').Fixture} */
 	let fixture;
 
 	before(async () => {
 		fixture = await loadFixture({
 			root: './fixtures/astro-envs/',
 		});
-
-		await fixture.build();
-	});
-
-	it('builds without throwing', async () => {
-		expect(true).to.equal(true);
-	});
-
-	it('does render public env and private env', async () => {
-		let indexHtml = await fixture.readFile('/index.html');
-
-		expect(indexHtml).to.include('CLUB_33');
-		expect(indexHtml).to.include('BLUE_BAYOU');
-	});
-
-	it('does render destructured public env and private env', async () => {
-		let indexHtml = await fixture.readFile('/destructured/index.html');
-
-		expect(indexHtml).to.include('CLUB_33');
-		expect(indexHtml).to.include('BLUE_BAYOU');
-	});
-
-	it('does render builtin SITE env', async () => {
-		let indexHtml = await fixture.readFile('/index.html');
-		expect(indexHtml).to.include('http://example.com');
-	});
-
-	it('does render destructured builtin SITE env', async () => {
-		let indexHtml = await fixture.readFile('/destructured/index.html');
-
-		expect(indexHtml).to.include('http://example.com');
 	});
 
-	it('includes public env in client-side JS', async () => {
-		let dirs = await fixture.readdir('/');
-		let found = false;
-
-		// Look in all of the .js files to see if the public env is inlined.
-		// Testing this way prevents hardcoding expected js files.
-		// If we find it in any of them that's good enough to know its working.
-		await Promise.all(
-			dirs.map(async (path) => {
-				if (path.endsWith('.js')) {
-					let js = await fixture.readFile(`/${path}`);
-					if (js.includes('BLUE_BAYOU')) {
-						found = true;
+	describe('Build', () => {
+		before(async () => {
+			await fixture.build();
+		});
+	
+		it('builds without throwing', async () => {
+			expect(true).to.equal(true);
+		});
+	
+		it('does render public env and private env', async () => {
+			let indexHtml = await fixture.readFile('/index.html');
+	
+			expect(indexHtml).to.include('CLUB_33');
+			expect(indexHtml).to.include('BLUE_BAYOU');
+		});
+	
+		it('does render destructured public env and private env', async () => {
+			let indexHtml = await fixture.readFile('/destructured/index.html');
+	
+			expect(indexHtml).to.include('CLUB_33');
+			expect(indexHtml).to.include('BLUE_BAYOU');
+		});
+	
+		it('does render builtin SITE env', async () => {
+			let indexHtml = await fixture.readFile('/index.html');
+			expect(indexHtml).to.include('http://example.com');
+		});
+	
+		it('does render destructured builtin SITE env', async () => {
+			let indexHtml = await fixture.readFile('/destructured/index.html');
+	
+			expect(indexHtml).to.include('http://example.com');
+		});
+	
+		it('does render builtin BASE_URL env', async () => {
+			let indexHtml = await fixture.readFile('/index.html');
+			expect(indexHtml).to.include('/blog');
+		});
+	
+		it('includes public env in client-side JS', async () => {
+			let dirs = await fixture.readdir('/');
+			let found = false;
+	
+			// Look in all of the .js files to see if the public env is inlined.
+			// Testing this way prevents hardcoding expected js files.
+			// If we find it in any of them that's good enough to know its working.
+			await Promise.all(
+				dirs.map(async (path) => {
+					if (path.endsWith('.js')) {
+						let js = await fixture.readFile(`/${path}`);
+						if (js.includes('BLUE_BAYOU')) {
+							found = true;
+						}
 					}
-				}
-			})
-		);
-
-		expect(found).to.equal(true, 'found the public env variable in the JS build');
+				})
+			);
+	
+			expect(found).to.equal(true, 'found the public env variable in the JS build');
+		});
+	
+		it('does not include private env in client-side JS', async () => {
+			let dirs = await fixture.readdir('/');
+			let found = false;
+	
+			// Look in all of the .js files to see if the public env is inlined.
+			// Testing this way prevents hardcoding expected js files.
+			// If we find it in any of them that's good enough to know its NOT working.
+			await Promise.all(
+				dirs.map(async (path) => {
+					if (path.endsWith('.js')) {
+						let js = await fixture.readFile(`/${path}`);
+						if (js.includes('CLUB_33')) {
+							found = true;
+						}
+					}
+				})
+			);
+	
+			expect(found).to.equal(false, 'found the private env variable in the JS build');
+		});
 	});
 
-	it('does not include private env in client-side JS', async () => {
-		let dirs = await fixture.readdir('/');
-		let found = false;
+	describe('Development', () => {
+		/** @type {import('./test-utils').DevServer} */
+		let devServer;
+		before(async () => {
+			devServer = await fixture.startDevServer();
+		});
+		after(async () => {
+			await devServer.stop();
+		});
 
-		// Look in all of the .js files to see if the public env is inlined.
-		// Testing this way prevents hardcoding expected js files.
-		// If we find it in any of them that's good enough to know its NOT working.
-		await Promise.all(
-			dirs.map(async (path) => {
-				if (path.endsWith('.js')) {
-					let js = await fixture.readFile(`/${path}`);
-					if (js.includes('CLUB_33')) {
-						found = true;
-					}
-				}
-			})
-		);
+		it('does render builtin BASE_URL env', async () => {
+			let res = await fixture.fetch('/blog/');
+			expect(res.status).to.equal(200);
+			let indexHtml = await res.text();
+			let $ = cheerio.load(indexHtml);
+			expect($('#base-url').text()).to.equal('/blog/');
+		});
 
-		expect(found).to.equal(false, 'found the private env variable in the JS build');
+		it('does render destructured builtin SITE env', async () => {
+			let res = await fixture.fetch('/blog/destructured/');
+			expect(res.status).to.equal(200);
+			let indexHtml = await res.text();
+			let $ = cheerio.load(indexHtml);
+			expect($('#base-url').text()).to.equal('/blog/');
+		});
 	});
 });
diff --git a/packages/astro/test/fixtures/astro-envs/astro.config.mjs b/packages/astro/test/fixtures/astro-envs/astro.config.mjs
index 7bb1b85f1d15..1fee9eb36b57 100644
--- a/packages/astro/test/fixtures/astro-envs/astro.config.mjs
+++ b/packages/astro/test/fixtures/astro-envs/astro.config.mjs
@@ -4,5 +4,6 @@ import vue from '@astrojs/vue';
 // https://astro.build/config
 export default defineConfig({
 	site: 'http://example.com',
+	base: '/blog',
 	integrations: [vue()],
 });
diff --git a/packages/astro/test/fixtures/astro-envs/src/components/Client.vue b/packages/astro/test/fixtures/astro-envs/src/components/Client.vue
index 7162c563295a..e7b79c0f74bb 100644
--- a/packages/astro/test/fixtures/astro-envs/src/components/Client.vue
+++ b/packages/astro/test/fixtures/astro-envs/src/components/Client.vue
@@ -10,6 +10,7 @@ export default {
     return {
       PUBLIC_PLACE: import.meta.env.PUBLIC_PLACE,
 			SECRET_PLACE: import.meta.env.SECRET_PLACE,
+			BASE_URL: import.meta.env.BASE_URL,
     };
   },
 };
diff --git a/packages/astro/test/fixtures/astro-envs/src/pages/destructured.astro b/packages/astro/test/fixtures/astro-envs/src/pages/destructured.astro
index e85b9bf81d2a..6c2c803fcd23 100644
--- a/packages/astro/test/fixtures/astro-envs/src/pages/destructured.astro
+++ b/packages/astro/test/fixtures/astro-envs/src/pages/destructured.astro
@@ -4,3 +4,4 @@ const { PUBLIC_PLACE, SECRET_PLACE, SITE } = import.meta.env;
 <environment-variable>{PUBLIC_PLACE}</environment-variable>
 <environment-variable>{SECRET_PLACE}</environment-variable>
 <environment-variable>{SITE}</environment-variable>
+<environment-variable id="base-url">{import.meta.env.BASE_URL}</environment-variable>
diff --git a/packages/astro/test/fixtures/astro-envs/src/pages/index.astro b/packages/astro/test/fixtures/astro-envs/src/pages/index.astro
index 3c265dbef736..f56c3cbc135a 100644
--- a/packages/astro/test/fixtures/astro-envs/src/pages/index.astro
+++ b/packages/astro/test/fixtures/astro-envs/src/pages/index.astro
@@ -4,4 +4,5 @@ import Client from '../components/Client.vue';
 <environment-variable>{import.meta.env.PUBLIC_PLACE}</environment-variable>
 <environment-variable>{import.meta.env.SECRET_PLACE}</environment-variable>
 <environment-variable>{import.meta.env.SITE}</environment-variable>
+<environment-variable id="base-url">{import.meta.env.BASE_URL}</environment-variable>
 <Client client:load />