The distinctive hump on the forehead of large individuals of the Humphead Wrasse, Cheilinus undulatus, a protogynous hermaphroditic reef fish, is often assumed to be a male-associated secondary sexual characteristic, but this has never been evaluated. A unique opportunity arose to obtain confiscated specimens, 161 females and 17 males, ranging in body sizes from 20.8 to 129.0 cm total length (TL), making this evaluation possible for the first time. The hump was quantified in terms of its maximum angle of elevation, and its development was determined to be size-associated. A notable hump was recorded in 16 females and all 17 males, becoming visible around 37.0 cm TL, and all specimens ≥75.0 cm TL exhibited a distinctive hump, irrespective of sex. There was a linear and positive correlation between body size and maximum angle of hump elevation for both sexes; males tended to have larger humps because they were larger in body size rather than because of sexual dimorphism for hump development. The development of a cephalic hump or a horn on the forehead has been documented in several groups of reef fishes, and its possible adaptive significance in different groups is discussed.