In Rome's Regal period (753–509 BC), the area across the Tiber belonged to the hostile Etruscans: the Romans named it Ripa Etrusca (Etruscan bank). Rome conquered it to gain control of and access to the river from both banks, but was not interested in building on that side of the river. In fact, the only connection between Trastevere and the rest of the city was a small wooden bridge called the Pons Sublicius (Latin: "bridge built on wooden piles").
By the time of the Republic c. 509 BC, the number of sailors and fishermen making a living from the river had increased, and many had taken up residence in Trastevere. Immigrants from the East also settled there, mainly Jews and Syrians. The area began to be considered part of the city under Augustus, who divided Rome into 14 regions (regiones in Latin); modern Trastevere was the XIV and was called Trans Tiberim.
Since the end of the Roman Republic the quarter was also the center of an important Jewish community, which inhabited there until the end of the Middle Ages.
With the wealth of the Imperial Age, several important figures decided to build their villae in Trastevere, including Clodia, (Catullus' "friend") and Julius Caesar (his garden villa, the Horti Caesaris). The regio included two of the most ancient churches in Rome, the Titulus Callixti, later called the Basilica di Santa Maria in Trastevere, and the Titulus Cecilae, Santa Cecilia in Trastevere.
In order to have a stronghold on the right Bank and to control the Gianicolo hill, Transtiberim was partially included by Emperor Aurelian (270–275) inside the wall erected to defend the city against the Germanic tribes.