St Illtyd's Church is a church complex in Llantwit Major, Vale of Glamorgan, southeast Wales. It is located at the site of the oldest college in the United Kingdom, once believed to have been founded as Côr Tewdws c. AD 395 by the Roman Emperor Theodosius II. It is now generally accepted as having been founded by St. Illtud c. AD 508, from whom it derives its name. The current church building was built in the 11th century by the Normans, with portions being rebuilt in the 13th and 15th centuries. The church building is one of the oldest and best-known parish churches in Wales. It is a grade I listed building, or building of exceptional interest, and has been called both the "Westminster Abbey of Wales" for its unique collection of carved stones and effigies, and "the most beautiful church in Wales."

Llantwit Major is a town and community in Wales on the Bristol Channel coast. It is one of four towns in the Vale of Glamorgan, with the third largest population after Barry and Penarth, and ahead of Cowbridge. It is 4+1⁄2 miles (7.2 km) from Cowbridge, 9 miles (14 km) from Bridgend, 10 miles (16 km) from Barry, and 15 miles (24 km) from Cardiff. It had a population of 9,530 in 2021.