A monopteros (Ancient Greek: ὁ μονόπτερος, from: μόνος, 'only, single, alone', and τὸ πτερόν, 'wing') is a circular colonnade supporting a roof but without any walls.^[1] Unlike a tholos (in its wider sense as a circular building), it does not have walls making a cella or room inside. In Greek and especially Roman antiquity, the term could also be used for a tholos. In ancient times, monopteroi (Ancient Greek: οἱ μονόπτεροι) served among other things as a form of baldachin for a cult image. An example of this is the Monument of Lysicrates in Athens, albeit with the spaces between the columns being walled in, even in ancient times. The Temple of Roma and Augustus on the Athenian Acropolis is a monopteros from Roman times, with open spaces between the columns. Cyriacus of Ancona, a 15th-century traveller, handed down his^[whose?] architrave inscription: Ad praefatae Palladis Templi vestibulum.^{[citation needed]}

A monopteros (or monopteron) is also known as cyclostyle (from the Greek words for "circle" and "column").^[2] The adjectives are monopteral and cyclostylar.

In Baroque and Neoclassical architecture, a monopteros as a pavilion, often given a classical name such as a "muses' temple" is a popular garden feature in English and French gardens. The monopteros also occurs in German parks, as in the English Garden in Munich and in Hayns Park in Eppendorf, Hamburg. Many wells in parks and spa centres are covered by a monopteros. Many monopteroi have staffage structures like a porticus placed in front of the monopteros. These also have only a decorative function, because they are not needed in order to provide an entrance to a temple that is open on all sides.

Many monopteroi are described as rotundas due to their circular floor plan. The tholos also goes by that name. However, many monopteroi have square or polygonal plans, and these would not be described as rotundas. An example is the Muses' Temple with the muse, Polyhymnia, in the grounds of Tiefurt House, that has a hexagonal floor plan.