Koine (meaning "common" or "shared" in Greek, and pronounced koy-NEH) is a Greek dialect that took its rise in the beginning of the Hellenistic era (the beginning of the 3rd century BC), and at first centered mainly around Alexandria in Egypt, but then spread throughout the Greek-speaking world from Gaul to Mesopotamia. Koine continued to be to be widespread until the end of the ancient world (around the 6th century AD).

Spoken Koine consisted of the spoken form of Attic, mixed with many Ionic words and forms and quite a few loans from other dialects and local languages (which contributed to the large veriety in Koine), but always with Attic orthography.

Literary Koine, which was a compromise between Attic and the spoken Koine, was an artificial and almost completely static dialect, from which spoken language drew further and further apart.

There are many writings in Koine, including the writings of the historians Polybius, Diodorus, Plutarch, Arrian, Cassius Dio and Josephus, the rhetorician Diodorus of Halicarnassus, and the geographer Strabo.

In the 2nd century AD, a reactionary movement (the "Atticists") to Koine, which advocated the use of pure Attic, was created.

Some researchers distinguish the Hellenistic as a sub-dialect of the Koine, by which they mean the language of the Septuagint and the New Testament. However, no accurate distiction can be made between Koine and Hellenistic.