Ionic Greek was a sub-dialect of the so called Attic-Ionic dialectal group of the ancient Greek language, which was itself a member of the Greek branch of the Indo-European language family. The other dialectal groups of ancient Greek were Doric, Aeolic, Arcado-Cypriot and Northwestern .

Ionic Greek was mainly spoken in the Greek colonies of Asia Minor. It was the language in which Herodotus wrote his famous History, since he was a native of Halicarnassos, a Greek colony in Asia Minor. Many Ionic traits are also found in the - mainly Aeolic - language of Homer's epic poetry.

Its main differences from standard ancient Greek or Attic was the following:

1. Attic long [a:] was turned into [e:] in every position, even after a vowel or [r], something which was not the case in Attic, e.g. Attic [nea:nia:s]> Ionic [nee:nie:s] = a young person.
2. In many cases, Attic initial aspiration lacked in Ionic, e.g. Attic [hippos]> Ionic [ikkos] = a horse.
3. In many cases Ionic turned ur-Greek labiovelar sound [qu] in [k] rather than [p] in front of back vowels, e.g. Attic [hopo:s]> Ionic [oko:s] = the same way (as). Attic a [p] in front of back vowels (i.e. [a], [o], [u]) and a [t], as a rule, in front of front vowels (i.e. [e], [i]), whereas Aeolic had a [p] everywhere. It is worth mentioning that the same difference existed also in Celtic and Italic branches of the Indo-European language family, for example between Latin and Oscan, as well as between P-Celtic (Welsh) and Q-Celtic (Irish), something which may point to a close relationship between these three Indo-European branches.
4. Ionic retained the original Greek hiatus (after the disappearance of some intervocalic consonants), whereas Attic turned it in a long vowel, according the case, e.g. Attic [gene:]> Ionic [genea]= genders, families.
5. Ionic retained the double [ss], as old Attic also did ; around 4th century B.C., Attic changed the aforementioned [ss] in [tt], e.g. Attic [tettares]> old Attic [tessares], the same as ionic [tessares]= four. Modern Greek, derived from the Koine (i.e. the "common" Greek language), returned to that older [ss] (nowadays pronounces as a single [s]), a phenomenon that was the result of many Ionic trait absorbed in Attic of the Hellenistic times.
6. Very analytical word-order, perhaps the most analytical one within ancient Greek dialects.