The adult morphology is described and illustrated of Neoplagioporus kajika sp. n. (Digenea: Opecoelidae) found in the Japanese fluvial sculpin Cottus pollux Günther (Osteichthyes: Scorpaeniformes: Cottidae) collected in the Naka River at Terase Bridge, Narutake, Nakagawa Town, Fukuoka Prefecture, Kyushu, Japan. This new species is characterized by that the body shape is oval, that the intestinal caeca end posteriorly at the middle level of the testicular region, that the ovary is trilobed, and that the vitelline follicles are distributed between the pharyngeal level and usually the posterior end of body and fill up the lateral fields of body. The new species is different from three hitherto known Neoplagioporus species, N. zacconis (Yamaguti, 1934) Shimazu, 1990 (type species), N. ayu (Takahashi, 1928) Shimazu, 1990, and N. elongatus (Goto et Ozaki, 1930) Shimazu, 1990, in a combination of these characteristics. The new species is considered mainly infective to C. pollux in the river.