The city was founded around 315 BC by the King Cassander of Macedon, on or near the site of the ancient town of Therma and 26 other local villages. He named it after his wife Thessalonike, a half-sister of Alexander the Great and princess of Macedon as daughter of Philip II. Under the kingdom of Macedon the city retained its own autonomy and parliament and evolved to become the most important city in Macedon.
After the fall of the kingdom of Macedon in 168 BC, Thessalonica became a free city of the Roman Republic under Mark Antony in 41 BC. It grew to be an important trade-hub located on the Via Egnatia, the road connecting Dyrrhachium with Byzantium, which facilitated trade between Thessaloniki and great centers of commerce such as Rome and Byzantium. Thessaloniki also lay at the southern end of the main north-south route through the Balkans along the valleys of the Morava and Axios river valleys, thereby linking the Balkans with the rest of Greece. The city later became the capital of one of the four Roman districts of Macedonia. Later it became the capital of all the Greek provinces of the Roman Empire due to the city's importance in the Balkan peninsula. When the Roman Empire was divided into the tetrarchy, Thessaloniki became the administrative capital of one of the four portions of the Empire under Galerius Maximianus Caesar, where Galerius commissioned an imperial palace, a new hippodrome, a triumphal arch and a mausoleum among others.
In 379 when the Roman Prefecture of Illyricum was divided between the East and West Roman Empires, Thessaloniki became the capital of the new Prefecture of Illyricum. With the Fall of Rome in 476, Thessaloniki became the second-largest city of the Eastern Roman Empire. Around the time of the Roman Empire Thessaloniki was also an important center for the spread of Christianity; the First Epistle to the Thessalonians written by Paul the Apostle is the first written
