Development of the sea otter placenta is described from implantation to term. It is a typical carnivore placenta, labyrinthine, endotheliochorial, and zonary (sometimes incomplete mesometrially). The annulus has 4 to 8 lobes and is widest antimesometrially. Maternal capillaries of the labyrinth have very thick almost columnar endothelium, and thick diastase resistant, heavily PAS positive basement membranes. They are surrounded by syntrophoblast and a few cytotrophoblast cells. Instances of intratrophoblastic fetal capillaries increase with age; thus maternal and fetal blood are in greater proximity in late gestation. The hematome arises during the limb‐bud period at the center of the antimesometrial portion of the developing chorio‐allantoic placenta as small isolated pouches of vascular chorio‐allantoic membrane filled with maternal blood. Each pouch is lined by columnar cytotrophoblast which phagocytoses maternal erythrocytes, and no doubt also absorbs histiotrophic products of the endometrial glands at its base. These early pouches grow and coalesce to form larger ones. The definitive hematome consists of one large, often bilobed sac (diam. 5 to 7 cm) surrounded at the base of its stalk by several smaller sacs. Elaborate villous folds subdivide the sacs and stalk. The whole hematomic mass invaginates the allantoic vesicle, the endoderm of which thus becomes its outer covering. The hematome remains very large to term. The uterine glands of the placental base are highly active throughout gestation.