diff --git a/packages/nuxt/src/app/composables/fetch.ts b/packages/nuxt/src/app/composables/fetch.ts
index e4f5d003d4..71e4405ce1 100644
--- a/packages/nuxt/src/app/composables/fetch.ts
+++ b/packages/nuxt/src/app/composables/fetch.ts
@@ -60,6 +60,7 @@ export function useFetch<
   if (!request) {
     throw new Error('[nuxt] [useFetch] request is missing.')
   }
+
   const key = _key === autoKey ? '$f' + _key : _key
 
   const _request = computed(() => {
@@ -70,6 +71,10 @@ export function useFetch<
     return unref(r)
   })
 
+  if (!opts.baseURL && typeof _request.value === 'string' && _request.value.startsWith('//')) {
+    throw new Error('[nuxt] [useFetch] the request URL must not start with "//".')
+  }
+
   const {
     server,
     lazy,