Gasterosteus kamoensis sp. nov. is described from the Upper Miocene Minamiimogawa Formation in Kamo City, Niigata, Japan. The new species represents the oceanic complete morph. It is characterized by: short and high ectocoracoid; presence of posttemporal bone; fully developed pelvic girdle with convex anterior margin; wide and not tapering posterior process of pelvic bone; four dorsal spines; comparatively long spines in dorsal, pelvic and anal fins; and large body size, apparently exceeding 130 mm in standard length. Gasterosteus kamoensis sp. nov. is the third nominal fossil species of this genus described from East Asia, along with freshwater G. orientalis Sytchevskaya and the marine G. abnormis Gretchina. Together, these discoveries demonstrate the wide distribution and morphological and ecological diversity of this genus during the Miocene in the northwestern Pacific.