diff --git a/frontend/components/auth/app-auth-guard.tsx b/frontend/components/auth/app-auth-guard.tsx
index 987e73b..382b9ed 100644
--- a/frontend/components/auth/app-auth-guard.tsx
+++ b/frontend/components/auth/app-auth-guard.tsx
@@ -1,28 +1,46 @@
 "use client";
 
 import { useRouter } from "next/navigation";
-import { type ReactNode, useEffect } from "react";
+import { type ReactNode, useEffect, useRef } from "react";
 
 import { authClient } from "@/lib/auth-client";
 
-// Authoritative client-side gate for the app shell: requires a session and an
-// active clinic, otherwise redirects to login / onboarding. The API enforces
-// the same rules server-side.
+// Authoritative client-side gate for the app shell. Requires a session and an
+// active clinic. If the user is signed in without an active clinic but already
+// belongs to one, we select it automatically; onboarding is only for users
+// with no clinics at all. The API enforces the same access rules server-side.
 export function AppAuthGuard({ children }: { children: ReactNode }) {
   const router = useRouter();
-  const { data, isPending } = authClient.useSession();
+  const { data: session, isPending } = authClient.useSession();
+  const { data: orgs, isPending: orgsPending } =
+    authClient.useListOrganizations();
+  const settingActive = useRef(false);
 
-  const ready = Boolean(data?.user && data.session?.activeOrganizationId);
+  const hasUser = Boolean(session?.user);
+  const activeOrgId = session?.session?.activeOrganizationId ?? null;
 
   useEffect(() => {
     if (isPending) return;
-    if (!data?.user) {
+    if (!hasUser) {
       router.replace("/login");
-    } else if (!data.session?.activeOrganizationId) {
+      return;
+    }
+    if (activeOrgId) return;
+
+    // Signed in but no active clinic selected yet.
+    if (orgsPending) return;
+    const first = orgs?.[0];
+    if (first) {
+      if (!settingActive.current) {
+        settingActive.current = true;
+        void authClient.organization.setActive({ organizationId: first.id });
+      }
+    } else {
       router.replace("/onboarding");
     }
-  }, [data, isPending, router]);
+  }, [isPending, hasUser, activeOrgId, orgsPending, orgs, router]);
 
+  const ready = hasUser && Boolean(activeOrgId);
   if (!ready) {
     return (
       <div className="flex h-dvh w-full items-center justify-center text-sm text-muted-foreground">