Located on the northeastern summit of the Viminal Hill, one of Rome's Seven Hills, the Baths of Diocletian were built within the Agger of the Servian Wall near modern-day Piazza della Repubblica and Termini rail station. They served the residents of the Viminal, Quirinal, and Esquiline quarters, supplied by the Aqua Marcia and possibly the Aqua Antoniniana aqueducts. The site includes remnants of private homes and monuments demolished for its construction and now houses structures like the Basilica of Santa Maria degli Angeli e dei Martiri and part of the National Roman Museum. The baths remained functional until the Ostrogoths severed Rome’s aqueducts during the siege of 537 CE.

From the central structure were derived the plans for the Basilica of Constantine and Maxentius. The Smithsonian's National Museum of Natural History building in Washington, D.C., was partially based on design elements from these baths, including its Diocletian windows.

circa 298 CE- 1884 CE

The Baths of Diocletian were commissioned by Emperor Maximian in 298 CE to honor his co-emperor, Diocletian, upon the latter’s return from Africa. Evidence supporting this attribution comes from bricks in the main areas of the baths, which bear distinctive stamps from the Diocletianic period. This indicates the monumental impact of the project on the brick industry, as production was redirected under imperial control. Construction began in 298 CE and continued until sometime between Diocletian's abdication in 305 CE and the death of Constantius in July 306 CE.

The baths underwent restoration in the early 5th century and remained in use until the Ostrogothic siege of Rome in 537 CE, during which King Vitiges severed the aqueducts, rendering the complex unusable. In the medieval period, the Mirabilia Urbis Romae, a guidebook to the wonders of Rome, referred to the baths as "Palatium Diocletiani".

In the 1560s, Pope Pius IV repurposed part of the ruins to commemorate Christian martyrs said to have perished during the baths’ construction. The church of Santa Maria degli Angeli e dei Martiri, along with an attached Carthusian charterhouse, was established on the site. Michelangelo was commissioned to design the church, integrating the structures of the frigidarium and tepidarium, and also planning the main cloister of the charterhouse. A smaller cloister was built adjacent to the presbytery, occupying part of the area that had once housed the natatio. After 1575, under Pope Gregory XIII, additional halls of the baths were converted into storage facilities for grain and oil for the city of Rome.

Following Rome's incorporation into the Kingdom of Italy, the area around the baths underwent significant transformation. In 1884, the Carthusians abandoned the charterhouse, and large-scale urban development ensued. Roma Termini station was constructed, along with the Ministry of the Economy, the Grand Hotel, and Palazzo Massimo. Architect Gaetano Koch designed buildings along the Piazza dell’Esedra (now Piazza della Repubblica), partially dismantling the original exedra. Via Cernaia bisected the western gymnasium from the baths' enclosure wall, remnants of which are now located on Via Parigi. In 1889, the Italian government established the Museo Nazionale Romano in the baths and the former charterhouse, marking the site’s transformation into a space of cultural preservation.

circa 298-306 CE

The Baths of Diocletian were a monumental feat of Roman engineering and design. One of the inscriptions around the main entrance, translated from Latin, proclaims the dedication of the baths by Diocletian and Maximian to their fellow emperors and the Roman people. It emphasizes the scale and grandeur of the project, noting that the baths were commissioned by Maximin on his return from Africa and consecrated in Diocletian's name. Although only fragments of this inscription remain, a full transcription was preserved by an 8th- or 9th-century pilgrim at Einsiedeln Abbey in Switzerland.

The bath complex covered an expansive 13 hectares (32 acres), similar in size to the Baths of Caracalla. The main entrance was located to the northeast, while the southwest was dominated by a large exedra, the outline of which remains visible today as Piazza della Repubblica. This exedra was flanked by two large buildings, likely used as libraries, which in turn connected to circular halls. One of these halls now houses the Church of San Bernardo, while the other is located at the start of Via del Viminale. The central block of the baths measured 280 meters by 160 meters (910 feet by 520 feet), covering an area of 10.85 acres—larger than the 6 acres of the Baths of Caracalla.

Within the central block, the baths were arranged along a single axis, with the frigidarium, tepidarium, and caldarium flanked symmetrically by other halls. Two open-air gymnasiums bordered the frigidarium, with the remains of the western gymnasium still visible on Via Cernaia. The caldarium was flanked by two octagonal halls. Despite being similar in size to the Baths of Caracalla, the Baths of Diocletian are believed to have had a much greater capacity. Some sources claim they could accommodate up to 3,000 people simultaneously, although this figure is disputed, as the method by which Olympiodorus arrived at this estimate is unclear. The scale and organization of the baths, with their larger rooms and expanded entrance areas, likely allowed for greater functionality and the accommodation of more bathers.

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National Roman Museum
The National Roman Museum in the Baths of Diocletian (Museo Nazionale Romano alle Terme Diocleziano), is one of the several branches of the National Roman Museum. The branch of the museum located in the Diocletian's Baths shows exhibits from the pre- and early history of Rome, with a focus on archaeological findings from the historic periods of Ancient Rome.

The museum is located in what is known as "Michelangelo's Cloister" and some other buildings that were part of the Carthusian monastery as well as several halls south of the eastern palestra. The former main entrance hall of the museum connects the 16th-century CE outer garden around a large Krater used as a fountain with the cloister. The Epigraphic Museum is located in modern premises. The prehistoric exhibits are on the first floor of the cloister colonnade. The cloister itself exhibits numerous pieces of statuary and serves as a lapidarium.

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Basilica of Saint Mary of the Angels and of the Martyrs
The Basilica of Saint Mary of the Angels and of the Martyrs (Santa Maria degli Angeli e dei Martiri) is a minor Catholic titular basilica and former Carthusian conventual church in Rome. It was constructed in the then-ruined frigidarium and tepidarium of the Roman Baths of Diocletian in the Piazza della Repubblica. Originally it was constructed in the sixteenth century CE following an original design by Michelangelo Buonarroti. Ove the following centuries a number of other architects and artists added to the church architecture and decorative elements. During the Kingdom of Italy (circa 1861–1946 CE), the church was primarily used for religious state functions.