Koine Greek (ἡ κοινὴ διάλεκτος, hē koinḕ diálektos, lit. 'the common dialect'), also variously known as Hellenistic Greek, common Attic, the Alexandrian dialect, Biblical Greek, Septuagint Greek or New Testament Greek, was the common supra-regional form of Greek spoken and written during the Hellenistic period, the Roman Empire, and the early Byzantine Empire. It evolved from the spread of Greek following the conquests of Alexander the Great in the fourth century BC, and served as the lingua franca of much of the Mediterranean region and the Middle East during the following centuries. It was based mainly on Attic and related Ionic speech forms, with various admixtures brought about through dialect levelling with other varieties.

Koine Greek included styles ranging from conservative literary forms to the spoken vernaculars of the time. As the dominant language of the Byzantine Empire, it developed further into Medieval Greek, which then turned into Modern Greek.

Literary Koine was the medium of much post-classical Greek literary and scholarly writing, such as the works of Plutarch and Polybius. Koine is also the language of the Septuagint (the 3rd century BC Greek translation of the Hebrew Bible), the Christian New Testament, and of most early Christian theological writing by the Church Fathers. In this context, Koine Greek is also known as "Biblical", "New Testament", "ecclesiastical", or "patristic" Greek. The Roman Emperor Marcus Aurelius wrote his private thoughts in Koine Greek in a work that is now known as Meditations. Koine Greek continues to be used as the liturgical language of services in the Greek Orthodox Church and in some Greek Catholic churches.

The English-language name Koine is derived from the Koine Greek term ἡ κοινὴ διάλεκτος (hē koinḕ diálektos), meaning "the common dialect". The Greek word κοινή (koinḗ) itself means "common". The word is pronounced /kɔɪˈneɪ/, /ˈkɔɪneɪ/, or /kiːˈniː/ in US English and /ˈkɔɪniː/ in UK English. The pronunciation of the word koine itself gradually changed from [koinéː] (close to the Classical Attic pronunciation [koi̯.nɛ̌ː]) to [cyˈni] (close to the Modern Greek [ciˈni]). In Modern Greek, the language is referred to as Ελληνιστική Κοινή, "Hellenistic Koiné", in the sense of "Hellenistic supraregional language").

Ancient scholars used the term koine in several distinct senses. In the 2nd century, scholars such as Apollonius Dyscolus and Aelius Herodianus maintained the term koine to refer to the Proto-Greek language, while others used it to refer to any vernacular form of Greek speech which differed somewhat from the literary language.

When Koine Greek became a language of literature by the 1st century BCE, some people distinguished two forms: written as the literary post-classical form (which should not be confused with Atticism) and the vernacular. Others chose to refer to Koine as "the dialect of Alexandria" or "Alexandrian dialect" (ἡ Ἀλεξανδρέων διάλεκτος), or even the universal dialect of its time. Modern classicists have often used the former sense.

Koine Greek arose as a common dialect within the armies of Alexander the Great. Under the leadership of Macedon, their newly formed common variety was spoken from the Ptolemaic Kingdom of Egypt to the Seleucid Empire of Mesopotamia. It replaced existing ancient Greek dialects with an everyday form that people anywhere could understand. Though elements of Koine Greek took shape in Classical Greece, the post-Classical period of Greek is defined as beginning with the death of Alexander the Great in 323 BC, when cultures under Greek sway in turn began to influence the language.

In its eastern provinces, the Roman Empire communicated almost exclusively in Greek, even in areas where Greek was not the dominant spoken language. Local elites and communities, including those in Asia Minor, Arabia, Judaea, Syria, and Egypt, used Greek in inscriptions, documents, and official announcements.