diff --git a/.changeset/cyan-vans-clap.md b/.changeset/cyan-vans-clap.md
new file mode 100644
index 000000000..e23c13896
--- /dev/null
+++ b/.changeset/cyan-vans-clap.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Fix `astro:build:setup` hook `updateConfig` utility, where the configuration wasn't correctly updated when the hook was fired.
diff --git a/packages/astro/src/core/build/static-build.ts b/packages/astro/src/core/build/static-build.ts
index a7d835450..7d6004c0d 100644
--- a/packages/astro/src/core/build/static-build.ts
+++ b/packages/astro/src/core/build/static-build.ts
@@ -207,7 +207,7 @@ async function ssrBuild(
 		base: settings.config.base,
 	};
 
-	await runHookBuildSetup({
+	const updatedViteBuildConfig = await runHookBuildSetup({
 		config: settings.config,
 		pages: internals.pagesByComponent,
 		vite: viteBuildConfig,
@@ -215,7 +215,7 @@ async function ssrBuild(
 		logging: opts.logging,
 	});
 
-	return await vite.build(viteBuildConfig);
+	return await vite.build(updatedViteBuildConfig);
 }
 
 async function clientBuild(
diff --git a/packages/astro/src/integrations/index.ts b/packages/astro/src/integrations/index.ts
index ae1ff9761..6e27920eb 100644
--- a/packages/astro/src/integrations/index.ts
+++ b/packages/astro/src/integrations/index.ts
@@ -286,7 +286,9 @@ export async function runHookBuildSetup({
 	pages: Map<string, PageBuildData>;
 	target: 'server' | 'client';
 	logging: LogOptions;
-}) {
+}): Promise<InlineConfig> {
+	let updatedConfig = vite;
+
 	for (const integration of config.integrations) {
 		if (integration?.hooks?.['astro:build:setup']) {
 			await withTakingALongTimeMsg({
@@ -296,13 +298,15 @@ export async function runHookBuildSetup({
 					pages,
 					target,
 					updateConfig: (newConfig) => {
-						mergeConfig(vite, newConfig);
+						updatedConfig = mergeConfig(updatedConfig, newConfig);
 					},
 				}),
 				logging,
 			});
 		}
 	}
+
+	return updatedConfig;
 }
 
 export async function runHookBuildSsr({
diff --git a/packages/astro/test/units/integrations/api.test.js b/packages/astro/test/units/integrations/api.test.js
new file mode 100644
index 000000000..919628da2
--- /dev/null
+++ b/packages/astro/test/units/integrations/api.test.js
@@ -0,0 +1,30 @@
+import { expect } from 'chai';
+import { runHookBuildSetup } from '../../../dist/integrations/index.js';
+
+describe('Integration API', () => {
+	it('runHookBuildSetup should work', async () => {
+		const updatedViteConfig = await runHookBuildSetup({
+			config: {
+				integrations: [
+					{
+						name: 'test',
+						hooks: {
+							'astro:build:setup'({ updateConfig }) {
+								updateConfig({
+									define: {
+										foo: 'bar',
+									},
+								});
+							},
+						},
+					},
+				],
+			},
+			vite: {},
+			logging: {},
+			pages: new Map(),
+			target: 'server',
+		});
+		expect(updatedViteConfig).to.haveOwnProperty('define');
+	});
+});