diff --git a/.changeset/sweet-apricots-kneel.md b/.changeset/sweet-apricots-kneel.md
new file mode 100644
index 0000000000..1a114ec4ae
--- /dev/null
+++ b/.changeset/sweet-apricots-kneel.md
@@ -0,0 +1,5 @@
+---
+'@backstage/core-plugin-api': patch
+---
+
+Add a note that the `fetchApi` utility should not be used on sign-in page implementations and similar.
diff --git a/packages/core-plugin-api/src/apis/definitions/FetchApi.ts b/packages/core-plugin-api/src/apis/definitions/FetchApi.ts
index b1a4dbbd0b..aba0e53bb7 100644
--- a/packages/core-plugin-api/src/apis/definitions/FetchApi.ts
+++ b/packages/core-plugin-api/src/apis/definitions/FetchApi.ts
@@ -37,6 +37,13 @@ export type FetchApi = {
  * This is a wrapper for the fetch API, that has additional behaviors such as
  * the ability to automatically inject auth information where necessary.
  *
+ * Note that the default behavior of this API (unless overridden by your org),
+ * is to require that the user is already signed in so that it has auth
+ * information to inject. Therefore, using the default implementation of this
+ * utility API e.g. on the `SignInPage` or similar, would cause issues. In
+ * special circumstances like those, you can use the regular system `fetch`
+ * instead.
+ *
  * @public
  */
 export const fetchApiRef: ApiRef<FetchApi> = createApiRef({