One of the largest and most distinctly marked of the beaked whales (5) (6) (7) (8), the strap-toothed whale is named for the unique and somewhat bizarre teeth of the adult male. In common with other beaked whales, only two teeth become well developed, one on each side of the lower jaw, but in the male strap-toothed whale these grow up and over the upper jaw, reaching to over 30 centimetres in length, and curling over the top of the jaw so that the mouth is clamped nearly shut (2) (3) (5) (7). Female and immature strap-toothed whales have no visible teeth, making them more difficult to distinguish from other beaked whale species (7) (9) (10).
The body of the strap-toothed whale is robust and spindle-shaped, with a small dorsal fin about two-thirds of the way down the body, small flippers, and an unnotched tail fluke with pointed tips (7). Unusually, the species shows the reverse of typical cetacean colouration, being dark below and lighter above (8). The body is mainly black, with a white throat and upper back, white front to the beak, a white patch around the genital area, and a dark mask over the eyes and melon (3) (5) (6) (10). The pattern of light and dark areas is reported to be reversed in juveniles (10).