Bassae (Latin: Bassae, Ancient Greek: Βάσσαι – Bassai, meaning "little vale in the rocks") is an archaeological site in Oichalia, a municipality in the northeastern part of Messenia, Greece. In classical antiquity, it was part of Arcadia. Bassae lies near the village of Skliros, northeast of Figaleia, south of Andritsaina and west of Megalopolis. It is famous for the well-preserved mid- to late-5th century BC Temple of Apollo Epicurius.

Although this temple is geographically remote from major polities of ancient Greece, it is one of the most studied ancient Greek temples because of its multitude of unusual features. Bassae was the first Greek site to be inscribed on the World Heritage List, in 1986.

The temple was dedicated to Apollo Epikourios ("Apollo the helper"). It sits at an elevation of 1,131 metres above sea level on the slopes of Kotylion Mountain. Its construction is placed between 450 BC and 400 BC; the plague its dedication related to was probably the plague of Athens of 430 BC. It was supposedly designed by Iktinos, architect at Athens of the Parthenon. Modern scholars such as A. W. Lawrence are doubtful about this, as though the Doric order used is "extremely similar" to that of the Parthenon, where they differ Bassae is "old-fashioned", yet it was probably built later.

The ancient writer Pausanias praises the temple as eclipsing all others in the Peloponnesos except the Temple of Athena Alea at Tegea by the beauty of its stone and the harmony of its construction.

Pausanias described it in the 2nd century AD:

It was in use until the 4th- or 5th-century AD, when all pagan temples were forcibly closed during the persecution of pagans in the late Roman Empire. Probably because of the remoteness of the site, the temple remained unusually well-preserved. Depictions by artists, which begin in the early 19th century, show the pteron or external colonnade standing complete, and the architrave nearly so. The internal walls were a mass of fallen blocks, but relatively few had been removed for reuse, the normal fate of these conveniently sized pieces.

The temple is aligned north–south, in contrast to the majority of Greek temples which are aligned east–west; its principal entrance is from the north. This was necessitated by the limited space available on the steep slopes of the mountain. To overcome this restriction a door was placed in the side of the temple, perhaps to let light in to illuminate the cult statue.

The temple is of a relatively modest size, with the stylobate measuring 38.3 by 14.5 metres containing a Doric peristyle of six by fifteen columns (hexastyle). It has been proposed that there was a central space in the roof left open to admit light and air but there is no proof of this feature. The temple was constructed entirely out of grey Arcadian limestone except for the Bassae Frieze which was carved from marble (probably in ancient times colored with paint). Like most major temples it has three "rooms" or porches: the pronaos, plus a naos and an opisthodomos. The naos may have housed a cult statue of Apollo, although it is also surmised that the single 'proto-Corinthian' capital discovered by Charles Robert Cockerell and subsequently lost at sea, may have topped the single column that stood in the centre of the naos, and have been intended as an aniconic representation of Apollo Borealis. The temple lacks some optical refinements found in the Parthenon, such as a subtly curved floor, though the columns have entasis.