Current knowledge of plesiosaurs of clade Cryptoclidia is constrained by a lack of fossils from outside the Oxford Clay deposits of England. Recent fleldwork in the Sundance Formation of the Bighorn Basin, Wyoming, has resulted in the recovery of significant new fossils of cryptoclidid plesiosaurs, including the small-bodied form Tatenectes laramiensis. A new partial skeleton of this taxon is reported here; it is the most complete and best-preserved example of the taxon found to date, comprising a complete dorsal vertebral series, many ribs and gastralia, and a complete pelvic girdle. This skeleton illuminates several unique features of the taxon, including a novel pattern of midline pachyostosis in the gastralia. In addition, a range of both axial and appendicular morphological features reveals that Tatenectes had a body shape unique among known plesiosaurs, characterized by extreme dorsoventral compression, and modest anteroposterior reduction. The combination of the new skeleton with information from previous finds allows the first reconstruction of the taxon. Tatenectes had a dorsoventrally compressed, oblate spheroid body shape, with a high skeletal mass concentration in the ventral elements. We hypothesize that these features were adaptations for increased near-surface stability, perhaps allowing access to above normal wave base, inshore environments in the shallow Sundance Seaway.