The Basilica of St. Mary of the Angels and the Martyrs (Latin: Beatissimae Virgini et omnium Angelorum et Martyrum, Italian: Santa Maria degli Angeli e dei Martiri) is a titular basilica church in Rome, built inside the frigidarium of the Baths of Diocletian in the Piazza della Repubblica. The Cardinal priest of the Titulus S. Mariae Angelorum in Thermis is William Henry Keeler.

The basilica is dedicated to the Christian martyrs, known and unknown. It was also a personal monument of Pope Pius IV, whose tomb is in the apsidal tribune that culminates the series of spaces.
The thermae of Diocletian dominated the Quirinal Hill with their ruined mass and had successfully resisted Christianization. Michelangelo Buonarroti worked from 1563 to 1564 to adapt a section of the remaining structure of the baths to enclose a church. Some later construction directed by Luigi Vanvitelli in 1749 only superficially distracts from the grand and harmonious Michelangelesque volumes. At Santa Maria degli Angeli, Michelangelo achieved an unexampled sequence of shaped architectural spaces with few precedents or followers. There is no true facade (illustration); the simple entrance is set within one of the coved apses of a main space of the thermae. The plan is developed from a Greek cross, with a transept so dominant, with its cubical chapels at each end, that the effect is of a transverse nave.
The vestibule with canted corners and identical side chapels leads to a second vestibule, repeated on the far side of the transept, dominated by the over lifesize Saint Bruno of Cologne by Jean Antoine Houdon (1766). The great vaulted transept gives a striking display of the magnificent scale of Roman constructions, 90.8 meters long, and with the floor that Michelangelo raised to bring it up to the Seicento street level, 28 meters high. Raising the floor truncated the red granite Roman columns (illustration on left) that articulate the transept and its flanking spaces. Michelangelo made the transept 27 meters wide, thus providing vast cubical spaces at each end of the transept.
In 2006, Polish-born sculptor Igor Mitoraj created new bronze doors as well as a statue of John the Baptist for the basilica.
In April 2010, a five metre high bronze statue of Galileo Galilei Divine Man (designed by 1957 Nobel laureate Tsung-Dao Lee) was unveiled in a courtyard within the complex. The statue (a dedication to the 17th century scientist and philosopher) was a donation from CCAST (China Center of Advanced Science and Technology) and WFS (World Federation of Scientists).

Santa Maria degli Angeli was the official state church during the Kingdom of Italy (1870-1946). More recently, national burials have been held in the church. The church hosts the tombs of General Armando Diaz and Admiral Paolo Thaon di Revel, who were the commanders responsible for winning World War I on the Italian front.Also today the Basilica is used for many ceremonies, included the funeral of soldiers killed aboard.