A rotunda (from Latin rotundus) is any roofed building with a circular ground plan, and sometimes covered by a dome. It may also refer to a round room within a building (an example being the one below the dome of the United States Capitol in Washington, D.C.). The Pantheon in Rome is perhaps the most famous, and is the most influential rotunda. A band rotunda is a circular bandstand, usually with a dome.

The terminology of Ancient Greek architecture and Roman architecture distinguishes between two types of rotunda: a tholos is enclosed by a wall, while a monopteros is just a circular colonnade with a roof (like a modern bandstand or park pavilion). It is not clear that any Greek example was actually a Greek temple, but several were Roman temples, though mostly much smaller than the Pantheon, and with very different designs. The Temple of Hercules Victor and Temple of Vesta in Rome, along with the Temple of Vesta, Tivoli, are the best known and best preserved examples.

The few large Greek tholoi had varied functions, not all of which are now clear. Several are at major religious sanctuaries, but seem not to have been conventional temples. At most only the foundations and a few columns remain in place. They include the Tholos of Delphi, the Philippeion at Olympia, a small memorial to the family of Philip of Macedon, and a large building at the Sanctuary of Asclepius, Epidaurus. The largest Greek tholos, of uncertain function, was built in the Samothrace temple complex in the 260s BC. It is often called the Arsinoeum, as a dedication tablet for the Ptolomeic Queen Arsinoe II of Egypt has survived. The sanctuary was a great Hellenistic centre of Greco-Roman mysteries and the building probably played some role in these.

The oldest, the Tholos of Athens, was a large and plain rotunda used as a dining hall, and perhaps more, by the city's ruling council.

Later, very large, Roman rotundas include the Castel Sant'Angelo in Rome, built in the 130s as a mausoleum for the emperor Hadrian, and in the Middle Ages turned into a castle, and the 4th-century Rotunda in Thessaloniki, probably also intended as an imperial mausoleum, but later used as a church and a mosque. The church of Santa Costanza in Rome is a circular funerary chapel of the 4th century, probably built for one or more of the daughters of Constantine the Great, originally placed next to a funerary hall that is now only a ruined wall.

Some of the earliest free-standing temples in India are thought to have been of a circular type, as the Buddhist Bairat Temple in Bairat, Rajasthan, formed of a central stupa surrounded by a circular colonnade and an enclosing wall, built during the time of Emperor Ashoka and near which were found several Minor Rock Edicts. Ashoka also built the Mahabodhi Temple in Bodh Gaya circa 250 BC, possibly also a circular structure, next to the Bodhi tree. Representations of this early temple structure are found on a 100 BCE relief from the stupa railing at Bhārhut, as well as in Sanchi. These circular-type temples were also found in later rock-hewn caves such as Tulja Caves or Guntupalli. Circularity in Buddhist architecture was generally to allow a path for pradakshina or devotional circling of a round and solid stupa.

It has been suggested that these circular structures with colonnades may have originated with the Greek circular tholos temple, as in the Tholos of Delphi, but circular wooden huts in India are a more likely source of inspiration.

The Pisa Baptistery is the outstanding late medieval rotunda, taking from 1152 to 1363 to build, and including Romanesque, Gothic and classicizing or Proto-Renaissance elements. There are a number of other round churches.