St. Gereon's Basilica, Romanesque basilica in Altstadt-Nord, Cologne, Germany

St. Gereon's Basilica is a Romanesque and Gothic church in Cologne's Altstadt-Nord district. Its core is a fourth-century oval central building, encased in 1219–1227 as a decagon with a ribbed dome, 21.0 m long and 16.9 m wide. Two tall square towers flank a rounded choir section with a band of small arches. Inside, pointed arches rise from clustered piers to pale ribbed vaulting, and stained-glass windows fill the apse.

The original oval building dates from the fourth century, making it one of Germany's oldest remaining sacred structures at its original location. Over the centuries, the building was expanded and redesigned multiple times, with the Romanesque and Gothic phases being particularly formative.

The church is dedicated to Saint Gereon and his Theban Legion; since 1121 his bones have been venerated as a relic here. Gregory of Tours, writing around 590, called the church ad sanctos aureos, the church of the golden saints. A granite column fragment by the entrance is linked by inscription to the legend of Gereon's blood column.

The church is accessible Monday through Saturday from 9:00 to 12:30, and Monday through Friday plus Sunday from 13:30 to 18:00. Visitors should note the midday break and plan accordingly to make use of both visiting windows.

The late antique oval dome measured 23.70 by 19.80 metres; the present decagon dome, completed in 1227, is 21.0 metres long and 16.9 metres wide. Hollow clay amphorae were built into the ancient vault to reduce its weight and speed the setting of the concrete. In the south conches, three bricked-up arched windows of the 4th-century building remain visible.