Trier was founded by the Romans in the late 1st century BCE as Augusta Treverorum. The city was the capital of the romanised Celtic tribe of the Treveri (Civitas Treverorum) and became an important economic and political center supporting the Roman defences on the Rhine further East. Over time its importance for the Romans rose further, first becoming the capital of the province of Belgic Gaul and later on the capital of the prefecture of the Gauls, which oversaw much of the Western Roman Empire.

In the late third and early fourth century, several emperors, including Constantine the Great, took up official residence in Trier. This caused the city to grow substantially in the fourth century, with its population reaching an estimated 80,000, while adding to its cityscape some of the monumental buildings that remain (in some form) in Trier to this day, including the Basilica of Constantine, an earlier version of the Cathedral and the Imperial Baths.

Following the Roman abandonment of their border defences on the Rhein in the fifth century, Trier was overrun by Frankish invaders from across the Rhine. During this period, the city lost some 90 percent of its population.