A fossil palatine of an alepisauroid fish collected from the middle Miocene Yokoo Formation in Nagano Prefecture, central Japan is described as Omosudis sp. Although the palatine is only a fragment, the palatine teeth arranged in a single row are well preserved. In fact, the palatine tooth characteristics are adequate for generic-level diagnosis. The fossil appears to be assignable to the genus Omosudis belonging to the family Alepisauridae by having the following characteristics: enormously large, posteriorly inclined teeth each with a sharply pointed apex, apico-basal striations, a nearly straight to arcuate anterior cutting edge, a wide pulp cavity surrounded by a thin dentine layer and a fang-like outline due to a basally elongated postapical barb. The Yokoo specimen represents the first reliable fossil record of the genus from the middle Miocene in Japan and appears to mark the earliest occurrence of this Recent genus in the Northwest Pacific region.