The Baths of Trajan were a massive thermae, a bathing and leisure complex, built in ancient Rome starting from 104 AD and dedicated during the Kalends of July in 109. Commissioned by Emperor Trajan, the complex of baths occupied space on the southern side of the Oppian Hill on the outskirts of what was then the main developed area of the city, although still inside the boundary of the Servian Wall. The architect of the complex is said to be Apollodorus of Damascus.
The baths were being utilized mainly as a recreational and social center by Roman citizens, both men and women, as late as the early 5th century. The complex seems to have been deserted soon afterwards as a cemetery dated to the 5th century (which remained in use until the 7th century) has been found in front of the northeastern exedra. The baths were thus no longer in use at the time of the siege of Rome by the Goths in 537; with the destruction of the Roman aqueducts, all thermae were abandoned, as was the whole of the now-waterless Mons Oppius.
Prior to the construction of the Baths, their location on the Oppian Hill was occupied by the ornate Palace of Nero (Domus Aurea). After Nero's suicide, subsequent emperors Vespasian, Titus, and Domitian chose to build over his palace with other forms of architecture. Emperor Trajan covered up the last of the palace with a platform upon which the Baths were built. Because they served as a model for bath complexes built throughout the Roman world during the Imperial period, these Baths would come to be recognized as a highly notable example of early Imperial Roman architecture.
The baths were erected on the Oppian Hill, a southern extension of the Esquiline Hill. Built on a platform that had itself been built over Nero's Palace, the bath complex was immense by ancient Roman standards, covering an area of approximately 330 by 215 meters. The complex rested on a northeast-southwest axis, with the main building attached to the northeast wall. This was contrary to the more widely used north-south axis of many buildings in the vicinity. It is suggested that this unorthodox orientation was chosen by the architects to reduce the bathers' exposure to the wind, while also maximizing exposure to the sun.