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, Cor Tewdws, which was founded c. 395 AD in honor of the Roman Emperor Theodosius I. It was refounded by St. Illtud c. 508 AD, 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.