The Middle Jurassic sauropod Cetiosaurus is significant both historically and in terms of its potential phylogenetic relationships. The anatomy and taxonomy of this form are poorly understood because inadequate diagnoses have allowed the proliferation of species names and the referral of very fragmentary specimens. A review of Cetiosaurus species indicates that all, except C. oxoniensis, are unavailable or nomina dubia. The current type species, C. medius, can no longer be regarded as a valid taxon. Previous suggestions that Cardiodon is a senior subjective synonym of Cetiosaurus cannot be sustained because the two forms do not share any autapomorphies. It is proposed that the generic name Cetiosaurus be retained, with C. oxoniensis as the new type species. The most complete specimen of C. oxoniensis (a partial skeleton from Bletchingdon Station, Oxfordshire) is redescribed and compared with other sauropods. Cetiosaurus is rediagnosed on the basis of autapomorphies, including: (1) ‘pyramid’-shaped neural spines in posterior cervical and anterior dorsal vertebrae; (2) loss of the spinodiapophyseal lamina on all dorsal vertebrae; (3) anterior chevrons with anteroposteriorly compressed distal shafts; (4) distal caudal centra have a ‘tongue’-like projection at the dorsal midline of their articular ends; and (5) a distinct triangular hollow on the lateral surface of the ilium at the base of the pubic process.