Ancient Greek is the historical stage in the development of the Greek language spanning the Archaic (c. 9th–6th centuries BC), Classical (c. 5th–4th centuries BC), and Hellenistic (c. 3rd century BC – 6th century AD) periods of ancient Greece and the ancient world. It is predated in the 2nd millennium BC by Mycenaean Greek. Its Hellenistic phase is known as Koine ("common") or Biblical Greek, and its late period mutates imperceptibly into Medieval Greek. Koine is regarded as a separate historical stage of its own, although in its earlier form it closely resembles Classical Greek. Prior to the Koine period, Greek of the classic and earlier periods included several regional dialects.

The Ancient Greek language is one of the most prominent in human cultural history, as it was the language of the works of Homer, of the historians, playwrights and philosophers during the Athenian Golden Age, and of the New Testament. It has made a large contribution to the vocabulary of English and was a standard subject of study in Western educational institutions from the Renaissance to the early 20th century. The New Latin used in the scientific binomial classification system continues today to draw vigorously from Ancient Greek vocabulary.

This article treats primarily the Archaic and Classical phases of the language – see also the articles on Mycenaean Greek and on Koine Greek.