The Ionic order (Greek ιωνικός ρυθμός) forms one of the three orders or organizational systems of classical architecture, the other two canonic orders being the Doric and the Corinthian. (There are two lesser orders, the stocky Tuscan order and the rich variant of Corinthian, the Composite order, added by 16th century Italian architectural theory and practice.)

The Ionic order originated in the mid-6th century BC in Ionia, the southwestern coastland and islands of Asia Minor settled by Ionian Greeks, where an Ionian dialect was spoken. The Ionic order column was being practiced in mainland Greece in the 5th century BC. The first of the great Ionic temples was the Temple of Hera on Samos, built about 570 BC–560 BC by the architect Rhoikos. It stood for only a decade before it was leveled by an earthquake. It was in the great sanctuary of the goddess: it could scarcely have been in a more prominent location for its brief lifetime. A longer-lasting 6th century Ionic temple was the Temple of Artemis at Ephesus, one of the Seven Wonders of the Ancient World.

The Ionic columns normally stand on a base which separates the shaft of the column from the stylobate or platform; The cap is usually enriched with egg-and-dart. Originally the volutes lay in a single plane (illustration at right); then it was seen that they could be angled out on the corners. This feature of the Ionic order made it more pliant and satisfactory than the Doric to critical eyes in the 4th century BC: angling the volutes on the corner columns, ensured that they "read" equally when seen from either front or side facade. The 16th-century Renaissance architect and theorist Vincenzo Scamozzi designed a version of such a perfectly four-sided Ionic capital; Scamozzi's version became so much the standard, that when a Greek Ionic order was eventually reintroduced, in the later 18th century Greek Revival, it conveyed an air of archaic freshness and primitive, perhaps even republican, vitality.