Acanthurus achilles has an elongated oval-shaped body with a steep dorsal profile to the head. The small, protrusible mouth is positioned low on the head with between 8 and 28 teeth fixed on each jaw, each having a flattened, serrated tip. The dorsal fin is supported by 9 spines and between 29 and 33 soft rays while the anal fin contains 3 spines and 26 to 29 soft rays. There is a single mobile spine on each side of the caudal peduncle which folds down into a slit. The caudal fin is concave. The overall colour is bluish-black with an irregular oval orange blotch on the rear of the body, a white bar on the gill cover, a wide orange bar on the caudal fin which also has a white posterior margin.[7] There is a pale blue ring around just inside the snout tip. This species reaches a maximum total length of 24 cm (9.4 in).[2]