The Erechtheion (latinized as Erechtheum /ɪˈrɛkθiəm, ˌɛrɪkˈθiːəm/; Ancient Greek: Ἐρέχθειον, Greek: Ερέχθειο) or Temple of Athena Polias is an ancient Greek Ionic temple-telesterion on the north side of the Acropolis, Athens, which was primarily dedicated to the goddess Athena. The building, made to house the statue of Athena Polias, has in modern scholarship been called the Erechtheion (the sanctuary of Erechtheus or Poseidon) in the belief that Pausanias' description of the Erechtheion applies to this building. However, whether the Erechtheion referred to by Pausanias is indeed the Ionic temple or an entirely different building has become a point of contention in recent decades.

In the official decrees the building is referred to as “... το͂ νεὸ το͂ ἐμ πόλει ἐν ο͂ι τὸ ἀρχαῖον ἄγαλμα” (the temple on the Acropolis within which is the ancient statue). In other instances it is referred to as the Temple of the Polias. The joint cult of Athena and Poseidon-Erechtheus appears to have been established on the Acropolis at a very early period, and they were even worshipped in the same temple as may, according to the traditional view, be inferred from two passages in Homer and also from later Greek texts. The extant building is the successor of several temples and buildings on the site. Its precise date of construction is unknown; it has traditionally been thought to have been built from circa 421–406 BC, but more recent scholarship favours a date in the 430s, when it could have been part of the programme of works instigated by Pericles.

The Erechtheion is unique in the corpus of Greek temples in that its asymmetrical composition doesn’t conform to the canon of Greek classical architecture. This is attributed either to the irregularity of the site, or to the evolving and complex nature of the cults which the building housed, or it is conjectured to be the incomplete part of a larger symmetrical building. Additionally, its post-classical history of change of use, damage and spoliation has made it one of the more problematic sites in classical archaeology. The precise nature and location of the various religious and architectural elements within the building remain the subject of debate. The temple was nonetheless a seminal example of the classical Ionic style, and was highly influential on later Hellenistic, Roman and Greek Revival architecture.