Holy Island or Holyhead Island, island in Gwynedd, north-western Wales, located in the northern St George's Channel, and connected by road and rail bridges to the island of Anglesey in the east. About 13 km (8 mi) by 5 km (3 mi), with an area of 39 sq km (16 sq mi), it is largely barren stone, rising to a high point at 219 m (719 ft).

The island's westerly location was noted by the Romans, who constructed a fort there in the 3rd century, inside which a church was built by St Cybi three centuries later. Holyhead is the island's only important settlement and, in addition to some engineering and tourism, its economy is centred on the important transport link between Ireland and central Britain, in operation since 1573.