This is a relatively slender unicornfish with the standard length being around three times the body's depth.

There is a single bony plate on each side of the caudal peduncle and this has a small, semicircular keel.

There are around 30 bluish horizontal stripes along the back and flanks, breaking into irregular shaped spots on the head.

[1] This fish is found on steep outward reef and lagoon slopes at depths of between 2 and 40 m (6 ft 7 in and 131 ft 3 in)[2] Naso thynnoides feeds on zooplankton and is typically found in small schools.

They are diurnal and at night they sleep on the reef, changing colour to adopt a disruptive camouflage.