Published anatomic studies of North American river otters (Lontra canadensis) include only comparative reviews of single anatomic systems, but this paper includes multiple clinically relevant anatomic features. Both captive and free-ranging North American river otters were studied by gross dissection of nine specimens, radiography of 18 live otters, and physical examination of 170 otters. Body fat in North American river otters is stored primarily subcutaneously at the base of the tail and in the axillae, so lack of visceral detail on abdominal radiographs is due to a lack of coelomic fat. The heart and great vessels are oriented as in the dog. The cardiac radiographic silhouette measures three intercostal spaces laterally between the fourth and sixth ribs and 4–4.5 intercostal spaces between the seventh through 11th ribs on the ventrodorsal projection. Subjective estimation of age by evaluation of tooth wear is confirmed by histologic examination of cementum annuli. Structures of the feet were imaged by high-resolution mammographic technique. The adult thymus was prominent, but the gland was small in the one juvenile animal studied. The spleen was unexpectedly large but consistent with findings in European otters (Lutra lutra). As in other otters and mammals, kidneys are reniculated.