diff --git a/.changeset/large-bananas-beg.md b/.changeset/large-bananas-beg.md
new file mode 100644
index 0000000000..0e22d385a6
--- /dev/null
+++ b/.changeset/large-bananas-beg.md
@@ -0,0 +1,5 @@
+---
+'@astrojs/starlight': patch
+---
+
+Fixes an i18n configuration issue for multilingual sites when using Astro’s `i18n` config with `prefixDefaultLocale` set to `false`.
diff --git a/packages/starlight/__tests__/basics/i18n.test.ts b/packages/starlight/__tests__/basics/i18n.test.ts
index 527783b9a0..32dd299aae 100644
--- a/packages/starlight/__tests__/basics/i18n.test.ts
+++ b/packages/starlight/__tests__/basics/i18n.test.ts
@@ -162,7 +162,7 @@ describe('processI18nConfig', () => {
 					locales: ['en', { codes: ['fr'], path: 'french' }],
 				},
 				expected: {
-					defaultLocale: { label: 'English', lang: 'en', dir: 'ltr', locale: 'en' },
+					defaultLocale: { label: 'English', lang: 'en', dir: 'ltr', locale: undefined },
 					locales: {
 						root: { label: 'English', lang: 'en', dir: 'ltr' },
 						french: { label: 'Français', lang: 'fr', dir: 'ltr' },
@@ -180,7 +180,7 @@ describe('processI18nConfig', () => {
 					routing: { prefixDefaultLocale: false },
 				},
 				expected: {
-					defaultLocale: { label: 'فارسی', lang: 'fa', dir: 'rtl', locale: 'fa' },
+					defaultLocale: { label: 'فارسی', lang: 'fa', dir: 'rtl', locale: undefined },
 					locales: {
 						root: { label: 'فارسی', lang: 'fa', dir: 'rtl' },
 						de: { label: 'Deutsch', lang: 'de', dir: 'ltr' },
diff --git a/packages/starlight/utils/i18n.ts b/packages/starlight/utils/i18n.ts
index 8cba4f5a2b..ce07b1cc4d 100644
--- a/packages/starlight/utils/i18n.ts
+++ b/packages/starlight/utils/i18n.ts
@@ -118,11 +118,12 @@ function getStarlightI18nConfig(
 		locales,
 		defaultLocale: {
 			...inferStarlightLocaleFromAstroLocale(defaultAstroLocale),
-			locale: isMonolingualWithRootLocale
-				? undefined
-				: isAstroLocaleExtendedConfig(defaultAstroLocale)
-					? defaultAstroLocale.codes[0]
-					: defaultAstroLocale,
+			locale:
+				isMonolingualWithRootLocale || (isMultilingual && !prefixDefaultLocale)
+					? undefined
+					: isAstroLocaleExtendedConfig(defaultAstroLocale)
+						? defaultAstroLocale.codes[0]
+						: defaultAstroLocale,
 		},
 	};
 }