diff --git a/app/lib/screens/study/onboarding/study_selection.dart b/app/lib/screens/study/onboarding/study_selection.dart
index 0f60e8ad7..6da27176e 100644
--- a/app/lib/screens/study/onboarding/study_selection.dart
+++ b/app/lib/screens/study/onboarding/study_selection.dart
@@ -300,6 +300,7 @@ class _InviteCodeDialogState extends State<InviteCodeDialog> {
                 .from('study_invite')
                 .select('preselected_intervention_ids')
                 .eq('code', _controller.text)
+                .eq('study_id', study.id)
                 .maybeSingle();
             if (!context.mounted) return;
             if (inviteResult != null &&
diff --git a/database/migration/20260506_scope_invite_codes_to_study.sql b/database/migration/20260506_scope_invite_codes_to_study.sql
new file mode 100644
index 000000000..5a375951f
--- /dev/null
+++ b/database/migration/20260506_scope_invite_codes_to_study.sql
@@ -0,0 +1,61 @@
+BEGIN;
+
+DO $$
+BEGIN
+  IF NOT EXISTS (
+    SELECT 1
+    FROM pg_constraint
+    WHERE conname = 'study_invite_study_id_code_unique'
+      AND conrelid = 'public.study_invite'::regclass
+  ) THEN
+    ALTER TABLE public.study_invite
+      ADD CONSTRAINT study_invite_study_id_code_unique UNIQUE (study_id, code);
+  END IF;
+END $$;
+
+ALTER TABLE public.study_subject
+  DROP CONSTRAINT IF EXISTS "study_subject_loginCode_fkey";
+
+ALTER TABLE public.study_subject
+  DROP CONSTRAINT IF EXISTS study_subject_study_invite_fkey;
+
+ALTER TABLE public.study_subject
+  ADD CONSTRAINT study_subject_study_invite_fkey
+  FOREIGN KEY (study_id, invite_code)
+  REFERENCES public.study_invite(study_id, code)
+  ON DELETE CASCADE;
+
+CREATE OR REPLACE FUNCTION public.is_invite_code_for_study(
+  study_id uuid,
+  invite_code text
+)
+RETURNS boolean
+LANGUAGE sql
+STABLE
+SECURITY DEFINER
+SET search_path = ''
+AS $$
+  SELECT EXISTS (
+    SELECT 1
+    FROM public.study_invite
+    WHERE study_invite.study_id = is_invite_code_for_study.study_id
+      AND study_invite.code = is_invite_code_for_study.invite_code
+  );
+$$;
+
+DROP POLICY IF EXISTS "Invite code must match study_id" ON public.study_subject;
+
+CREATE POLICY "Invite code must match study_id"
+ON public.study_subject
+AS RESTRICTIVE
+FOR INSERT
+TO authenticated
+WITH CHECK (
+  invite_code IS NULL
+  OR public.is_invite_code_for_study(study_id, invite_code)
+);
+
+REVOKE EXECUTE ON FUNCTION public.is_invite_code_for_study(uuid, text)
+FROM public, anon;
+
+COMMIT;
diff --git a/database/studyu-schema.sql b/database/studyu-schema.sql
index 90612a30b..7aaa7a395 100644
--- a/database/studyu-schema.sql
+++ b/database/studyu-schema.sql
@@ -212,6 +212,22 @@ $$;
 ALTER FUNCTION "public"."get_study_record_from_invite"("invite_code" "text") OWNER TO "postgres";
 
 
+CREATE OR REPLACE FUNCTION "public"."is_invite_code_for_study"("study_id" "uuid", "invite_code" "text") RETURNS boolean
+    LANGUAGE "sql" STABLE SECURITY DEFINER
+    SET "search_path" TO ''
+    AS $$
+  SELECT EXISTS (
+    SELECT 1
+    FROM public.study_invite
+    WHERE study_invite.study_id = is_invite_code_for_study.study_id
+      AND study_invite.code = is_invite_code_for_study.invite_code
+  );
+$$;
+
+
+ALTER FUNCTION "public"."is_invite_code_for_study"("study_id" "uuid", "invite_code" "text") OWNER TO "postgres";
+
+
 CREATE OR REPLACE FUNCTION "public"."handle_new_user"() RETURNS "trigger"
     LANGUAGE "plpgsql" SECURITY DEFINER
     SET "search_path" TO ''
@@ -611,6 +627,11 @@ ALTER TABLE ONLY "public"."study_invite"
 
 
 
+ALTER TABLE ONLY "public"."study_invite"
+    ADD CONSTRAINT "study_invite_study_id_code_unique" UNIQUE ("study_id", "code");
+
+
+
 ALTER TABLE ONLY "public"."study"
     ADD CONSTRAINT "study_pkey" PRIMARY KEY ("id");
 
@@ -660,7 +681,7 @@ ALTER TABLE ONLY "public"."study_invite"
 
 
 ALTER TABLE ONLY "public"."study_subject"
-    ADD CONSTRAINT "study_subject_loginCode_fkey" FOREIGN KEY ("invite_code") REFERENCES "public"."study_invite"("code") ON DELETE CASCADE;
+    ADD CONSTRAINT "study_subject_study_invite_fkey" FOREIGN KEY ("study_id", "invite_code") REFERENCES "public"."study_invite"("study_id", "code") ON DELETE CASCADE;
 
 
 
@@ -738,7 +759,7 @@ CREATE POLICY "Enable read access for study participants for fitbit credential"
 
 
 
-CREATE POLICY "Invite code must match study_id" ON "public"."study_subject" AS RESTRICTIVE FOR INSERT TO "authenticated" WITH CHECK ((("invite_code" IS NULL) OR ("study_id" IN ( SELECT ("public"."get_study_record_from_invite"("study_subject"."invite_code"))."id" AS "id"))));
+CREATE POLICY "Invite code must match study_id" ON "public"."study_subject" AS RESTRICTIVE FOR INSERT TO "authenticated" WITH CHECK ((("invite_code" IS NULL) OR "public"."is_invite_code_for_study"("study_id", "invite_code")));
 
 
 
@@ -848,6 +869,7 @@ REVOKE EXECUTE ON FUNCTION public.last_completed_task(uuid) FROM public, anon;
 
 -- RPC/API functions
 REVOKE EXECUTE ON FUNCTION public.get_study_record_from_invite(text) FROM public, anon;
+REVOKE EXECUTE ON FUNCTION public.is_invite_code_for_study(uuid, text) FROM public, anon;
 
 -- Trigger functions
 REVOKE EXECUTE ON FUNCTION public.handle_new_user() FROM public, anon;