The largetooth cookiecutter shark Isistius plutodus has no collar marking around throat, a small asymmetrical caudal fin with a short ventral lobe less than half the length of dorsal caudal margin, bigger mouth and gigantic lower teeth (proportionately the largest in any living shark) in 19 rows (upper teeth = 29 rows). Eyes set well forward on head, with extensive anterior binocular field. Pectoral fins rounded, pelvic fins smaller than dorsal fins.

As with the other member of the genus Isistius, it has a characteristic small cigar-shaped body with two small close-set spineless dorsal fins far posterior on back, no anal fin, huge, triangular-cusped teeth without blades, short, bulbous snout and a unique suctorial lips (Ref. 247).