The Church of St George (Ротонда „Свети Георги“ or Rotonda "Sveti Georgi") is an Early Christian red brick rotunda that is considered the oldest building in Sofia, the capital of Bulgaria. The rotunda has a diameter of 9.5m and is about 14m high. It's brick walls are an impressive 1.40m thick. The building is situated amid remains of the ancient town of Serdica.

The original function of the rotunda is still debated. Some say it was built as a bath (or kaldarium), others argue that since it was located in the centre of 2nd century AD Roman public buildings, it likely had a religious and ceremonial function. Emperor Constantine the Great turned the Rotunda into a Christian temple, which survived until the invasion of Attila's Huns, when it was almost completely destroyed. It was reconstructed during the reign of Emperor Justinian.