Adocus sengokuensis sp. nov. is described on the basis of disarticulated shell elements (nuchal, first peripherals, left fourth peripheral, left second costal, left hyoplastron, and right hypoplastron) collected from a lacustrine mudstone of the Lower Cretaceous Sengoku Formation, Kanmon Group in Miyawaka City, Fukuoka Prefecture, Japan. A. sengokuensis is characterized by its small size with a carapace estimated at only 29 cm long, a trapezoidal cervical scale greater in width than length, and a narrow lateral projection of the first pleural scale of the fourth peripheral. Small size and wide cervical scale suggest that this new species is the most basal taxon of the genus Adocus.