Trastevere (Italian: [trasˈteːvere]) is the 13th rione of Rome, Italy. It is identified by the initials R. XIII and it is located within Municipio I. Its name comes from Latin trans Tiberim (lit. 'beyond the Tiber').

Its coat of arms depicts a golden head of a lion on a red background, the meaning of which is uncertain.

In Rome's Regal period (753–509 BC), the area across the Tiber belonged to the 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 (English: 'bridge 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 lived there until the end of the Middle Ages. Rome's oldest remaining synagogue, though not used as such any longer, is found in the district. The building was constructed in 980, and became a synagogue in 1073 through the efforts of lexicographer Nathan ben Yechiel. Within the building there was also a mikveh. At the base of the central column Hebrew writing is still visible. Its use as a synagogue ended when the Jews were forced to move to the Roman ghetto on the other side of the Tiber river in the mid-16th century.^{[citation needed]} It is now used commercially, and can be found at 14, Vicolo dell’Atleta.

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.

In the Middle Ages Trastevere had narrow, winding, irregular streets; moreover, because of the mignani (structures on the front of buildings) there was no space for carriages to pass. At the end of the 15th century these mignani were removed. Nevertheless, Trastevere remained a maze of narrow streets. There was a strong contrast between the large, opulent houses of the upper classes and the small, dilapidated houses of the poor. The streets had no pavement until the time of Sixtus IV at the end of the 15th century. At first bricks were used, but these were later replaced by sampietrini (cobble stones), which were more suitable for carriages. Thanks to its partial isolation (it was "beyond the Tiber") and to the fact that its population had been multicultural since the ancient Roman period, the inhabitants of Trastevere, called Trasteverini, developed a culture of their own. In 1744 Benedict XIV modified the borders of the rioni, giving Trastevere its modern limits.