Acanthurus tristis was first formally described in 1993 by the American ichthyologist John Ernest Randall with its type locality given as Lively Rocks off Trincomalee on the east coast of Sri Lanka.

This is thought to be a reference to the grey, subdued "mournful" colour of adults.

The name was first used in an unpublished manuscript and painting by Samuel Tickell, subsequently being mentioned by Francis Day in 1888 who also described its ashy colour,[4] although Day classified Tickell's name as a synonym of Acanthurus tennenti.

The caudal fin also mimics that of C. eibli being black with a bright blue rear margin.

It is found from the Maldives and Chagos Archipelago east to the Andaman Sea and south to Java.