diff --git a/emulators/genesis_plus_gx.yml b/emulators/genesis_plus_gx.yml
index f844c792..d6f4389b 100644
--- a/emulators/genesis_plus_gx.yml
+++ b/emulators/genesis_plus_gx.yml
@@ -64,9 +64,9 @@ files:
   # -------------------------------------------------------
   - name: bios_U.sms
     system: sega-mastersystem
-    region: [north-america]
+    region: [north-america, europe]
     required: false
-    note: "Master System (NTSC-U) boot ROM. Snail maze game built in."
+    note: "Master System (NTSC-U) boot ROM. Snail maze game built in. REGION_USA is the default of sms_cart_region_detect for any export header outside the PAL CRC list, so European cartridges load it too (core/cart_hw/sms_cart.c:831-874)."
     source_ref: "core/loadrom.c:511"
 
   - name: bios_E.sms
diff --git a/emulators/genesis_plus_gx_wide.yml b/emulators/genesis_plus_gx_wide.yml
index dbb69a64..3d57f035 100644
--- a/emulators/genesis_plus_gx_wide.yml
+++ b/emulators/genesis_plus_gx_wide.yml
@@ -49,8 +49,9 @@ files:
 
   - name: bios_U.sms
     system: sega-mastersystem
-    region: [north-america]
+    region: [north-america, europe]
     required: false
+    note: "REGION_USA is the default of sms_cart_region_detect for any export header outside the PAL CRC list, so European cartridges load it too (core/cart_hw/sms_cart.c:863-864)."
     source_ref: "core/loadrom.c:512"
 
   - name: bios_E.sms
diff --git a/install/batocera.json b/install/batocera.json
index a75ff819..c78f7788 100644
--- a/install/batocera.json
+++ b/install/batocera.json
@@ -131004,6 +131004,7 @@
         "genesis_plus_gx_wide"
       ],
       "regions": [
+        "europe",
         "north-america"
       ],
       "region_groups": [
diff --git a/install/bizhawk.json b/install/bizhawk.json
index 51dcb329..57c376f2 100644
--- a/install/bizhawk.json
+++ b/install/bizhawk.json
@@ -8267,6 +8267,7 @@
         "sega-master-system"
       ],
       "regions": [
+        "europe",
         "north-america"
       ],
       "region_groups": [
diff --git a/install/lakka.json b/install/lakka.json
index 97794410..95d0ac4f 100644
--- a/install/lakka.json
+++ b/install/lakka.json
@@ -9081,6 +9081,7 @@
         "sega-master-system"
       ],
       "regions": [
+        "europe",
         "north-america"
       ],
       "region_groups": [
diff --git a/install/recalbox.json b/install/recalbox.json
index 0eb872e4..f615ad65 100644
--- a/install/recalbox.json
+++ b/install/recalbox.json
@@ -5602,6 +5602,7 @@
         "sega-master-system"
       ],
       "regions": [
+        "europe",
         "north-america"
       ],
       "region_system_groups": [
@@ -39284,6 +39285,7 @@
         "sega-master-system"
       ],
       "regions": [
+        "europe",
         "north-america"
       ],
       "region_groups": [
diff --git a/install/retroarch.json b/install/retroarch.json
index c6fbf48b..a3243acc 100644
--- a/install/retroarch.json
+++ b/install/retroarch.json
@@ -9200,6 +9200,7 @@
         "sega-master-system"
       ],
       "regions": [
+        "europe",
         "north-america"
       ],
       "region_groups": [
diff --git a/install/retrodeck.json b/install/retrodeck.json
index 2db379bd..204875d0 100644
--- a/install/retrodeck.json
+++ b/install/retrodeck.json
@@ -78082,6 +78082,7 @@
         "sega-master-system"
       ],
       "regions": [
+        "europe",
         "north-america"
       ],
       "region_groups": [
diff --git a/install/retropie.json b/install/retropie.json
index 9be53d31..ba7c1512 100644
--- a/install/retropie.json
+++ b/install/retropie.json
@@ -10145,6 +10145,7 @@
         "sega-master-system"
       ],
       "regions": [
+        "europe",
         "north-america"
       ],
       "region_groups": [
diff --git a/install/romm.json b/install/romm.json
index 6d04c924..a4ce77c6 100644
--- a/install/romm.json
+++ b/install/romm.json
@@ -5,7 +5,7 @@
   "platform": "romm",
   "display_name": "RomM",
   "version": "1.0",
-  "generated": "2026-10-18T11:04:01Z",
+  "generated": "2026-10-18T11:24:19Z",
   "base_destination": "bios",
   "detect": [
     {
@@ -3552,6 +3552,7 @@
         "sega-master-system"
       ],
       "regions": [
+        "europe",
         "north-america"
       ],
       "region_system_groups": [
diff --git a/tests/test_region.py b/tests/test_region.py
index d09c488f..8be6d0b7 100644
--- a/tests/test_region.py
+++ b/tests/test_region.py
@@ -937,5 +937,31 @@ class UnknownRegionSaysWhatToType(unittest.TestCase):
         self.assertIn("Short forms:", str(caught.exception))
 
 
+
+class GenesisPlusGxUsBiosServesEurope(unittest.TestCase):
+    """sms_cart_region_detect returns REGION_USA for any export header
+    outside the PAL CRC list, European cartridges included, and REGION_USA
+    loads bios_U.sms. Tagged north-america only, --region eu,jp dropped it."""
+
+    def test_eu_jp_keeps_bios_u(self):
+        from pathlib import Path
+
+        from common import load_emulator_profiles
+        from region import build_region_index, parse_requested, resolve_region_drops
+
+        root = Path(__file__).resolve().parents[1]
+        profiles = load_emulator_profiles(str(root / "emulators"))
+        for name in ("genesis_plus_gx", "genesis_plus_gx_wide"):
+            with self.subTest(profile=name):
+                index = build_region_index({name: profiles[name]})
+                group = {"sega-mastersystem": [
+                    (f"bios/{n}", n) for n in ("bios_U.sms", "bios_E.sms", "bios_J.sms")
+                ]}
+                drops = resolve_region_drops(
+                    group, index, parse_requested("eu,jp"), None
+                )
+                self.assertNotIn("bios/bios_U.sms", drops)
+
+
 if __name__ == "__main__":
     unittest.main()