The Baths of Diocletian were public baths in Rome, in what is now Italy. Named after emperor Diocletian and built from 298 to 306, they were the largest of the imperial baths. The project was originally commissioned by Maximian upon his return to Rome in the autumn of 298 and was continued after his and Diocletian's abdication under Constantius, father of Constantine.
The baths occupy the high-ground on the northeast summit of the Viminal, the smallest of the Seven hills of Rome, just inside the Agger of the Servian Wall. They served as a bath for the people residing in the Viminal, Quirinal, and Esquiline quarters of the city. The Quadrigae Pisonis, a 2nd-century monument with various reliefs, some private homes, and a relief representing the temple of Quirinus once stood at the site but were demolished to build the baths. The water supply was provided by the Aqua Marcia, an aqueduct that had long served the city of Rome since the early 2nd century. To properly supply the baths, the supply of water to the city was increased under the order of Diocletian. The baths may have also been supplied by the Aqua Antoniniana, which was originally positioned to supply Caracalla's baths in the early 3rd century.