Laricobius osakensis Montgomery and Shiyake sp. nov., collected from Adelges tsugae Annand on hemlock [Tsuga sieboldii Carr. and Tsuga diversifolia (Maxim.) Mast.] in Japan, is described and illustrated. The new species was collected from several localities on Honshu, Shikokou, and Kyushu Islands. The genus has not been reported previously from Japan. Morphological features, a molecular phylogeny, and diagnostic DNA sites are provided to distinguish this new species from previously described species of the genus. Because of plans to release L. osakensis for the biological control of A. tsugae in eastern North America, a key and discussion are provided to differentiate it from the native North American species, Laricobius nigrinus Fender, Laricobius laticollis Fall, and Laricobius rubidus LeConte, and from two previously imported species—Laricobius erichsonii Rosenhauer, which is endemic in Europe, and Laricobius kangdingensis Zilahi-Balogh & Jelinek, which is endemic in China.