The Polycotylidae are short-necked plesiosaurs known from the Cretaceous in various parts of the world, but only a few occurrences have been documented in Japan, where elasmosaurid remains are much more common. An indeterminate polycotylid specimen from the Upper Cretaceous in Obira Town, Hokkaido, is described. Characteristics of the vertebrae and clavicular arch support its taxonomic affinity. The Turonian occurrence of the specimen indicates the continuous presence of the Polycotylidae across the Cenomanian-Turonian boundary in the northwestern Pacific. Macroscopic osteological features of the vertebrae and clavicular arch indicate an advanced stage of ossification, and there are histological characteristics suggesting slowed growth. The osteoporotic-like condition implies a high degree of aquatic adaption.