diff --git a/.changeset/curvy-socks-rhyme.md b/.changeset/curvy-socks-rhyme.md
new file mode 100644
index 000000000000..485402efee0e
--- /dev/null
+++ b/.changeset/curvy-socks-rhyme.md
@@ -0,0 +1,5 @@
+---
+"astro": patch
+---
+
+Improves environment variables replacement in SSR
diff --git a/packages/astro/src/core/create-vite.ts b/packages/astro/src/core/create-vite.ts
index a3edff3b917c..4ce605b9f52d 100644
--- a/packages/astro/src/core/create-vite.ts
+++ b/packages/astro/src/core/create-vite.ts
@@ -149,9 +149,9 @@ export async function createVite(
 		root: fileURLToPath(settings.config.root),
 		envPrefix: settings.config.vite?.envPrefix ?? 'PUBLIC_',
 		define: {
-			'import.meta.env.SITE': settings.config.site
-				? JSON.stringify(settings.config.site)
-				: 'undefined',
+			'import.meta.env.SITE': stringifyForDefine(settings.config.site),
+			'import.meta.env.BASE_URL': stringifyForDefine(settings.config.base),
+			'import.meta.env.ASSETS_PREFIX': stringifyForDefine(settings.config.build.assetsPrefix),
 		},
 		server: {
 			hmr:
@@ -308,3 +308,7 @@ function isCommonNotAstro(dep: string): boolean {
 		)
 	);
 }
+
+function stringifyForDefine(value: string | undefined): string {
+	return typeof value === 'string' ? JSON.stringify(value) : 'undefined';
+}
diff --git a/packages/astro/src/vite-plugin-env/index.ts b/packages/astro/src/vite-plugin-env/index.ts
index 1958344e595c..c52a345be477 100644
--- a/packages/astro/src/vite-plugin-env/index.ts
+++ b/packages/astro/src/vite-plugin-env/index.ts
@@ -48,12 +48,6 @@ function getPrivateEnv(
 			}
 		}
 	}
-	privateEnv.SITE = astroConfig.site ? JSON.stringify(astroConfig.site) : 'undefined';
-	privateEnv.SSR = JSON.stringify(true);
-	privateEnv.BASE_URL = astroConfig.base ? JSON.stringify(astroConfig.base) : 'undefined';
-	privateEnv.ASSETS_PREFIX = astroConfig.build.assetsPrefix
-		? JSON.stringify(astroConfig.build.assetsPrefix)
-		: 'undefined';
 	return privateEnv;
 }
 
@@ -74,18 +68,6 @@ export default function envVitePlugin({ settings }: EnvPluginOptions): vite.Plug
 	return {
 		name: 'astro:vite-plugin-env',
 		enforce: 'pre',
-		config() {
-			return {
-				define: {
-					'import.meta.env.BASE_URL': astroConfig.base
-						? JSON.stringify(astroConfig.base)
-						: 'undefined',
-					'import.meta.env.ASSETS_PREFIX': astroConfig.build.assetsPrefix
-						? JSON.stringify(astroConfig.build.assetsPrefix)
-						: 'undefined',
-				},
-			};
-		},
 		configResolved(resolvedConfig) {
 			viteConfig = resolvedConfig;
 		},