St. Gereon's Basilica is a Roman Catholic church in Cologne, Germany, dedicated to Saint Gereon, and designated a minor basilica on June 25, 1920. The first mention of a church at the site, dedicated to St. Gereon, appears in 612. However, the building of the choir gallery, apse, and transepts occurred later, beginning under Archbishop Arnold II von Wied in 1151 and ending in 1227. It is one of twelve great churches in Cologne that were built in the Romanesque style.
St. Gereon has a highly irregular plan, the nave being covered by a decagonal oval dome, 21.0 m long and 16.9 m wide, completed in 1227 on the remains of Roman walls, which are still visible. It is the largest dome built in the West between the erection of the Hagia Sophia in the 6th century and the Duomo of Florence in the 15th century.
Ernst Seifert built an organ there in 1898. In the 20th century, the architect Andreas Dilthey worked on its interior.