Sofia is one of the most ancient cities in Europe, first known as the Thracian city of Serdica, named after the Thracian tribe Serdi. The Odrysi tribe, an ethnos with its own kingdom, settled in the region around 500 BC. The city was conquered by Philip of Macedon and remained under Macedonian rule briefly during the 4th Century BC, later passing under the rule of the Philip's son, Alexander the Great.

The Romans conquered Sofia around AD 29 and renamed it Ulpia Serdica, established as an administrative centre (municipium) during the reign of Emperor Trajan. The city expanded significantly, as fortifications, towers, public baths, administrative buildings, as well as a basilica and a large amphitheatre, sprung up. The Roman Emperor Diocletian divided the province of Dacia, which corresponded to present-day Romania and Moldova, into Dacia Mediterranea and Dacia Ripensis. Serdica became the capital of Dacia Mediterranea.

Over the next 150 years the city continued to grow and flourish, to the extent that Roman Emperor Constantine I (Constantine the Great) began to call it ‘my Rome’. Serdica boasted a magnificent urban design and an active social life. The ancient city flourished under the rule of Byzantine (Eastern Roman) Emperor Justinian I. However, the Huns ravaged the city in AD 447, but later, Justinian rebuilt it and named it Triaditsa, which remained under Byzantine rule until AD 809, although Slavic tribes often pillaged it. The name Triaditsa survives in the city’s chief administrative district.