The inoceramid bivalve Mytiloides ipuanus (Wellman, 1959) from the Lower Albian is reported for the first time in the Northwest Pacific and described in detail. This is the oldest record of a Cretaceous inoceramid in this region. It is possible that the present study documents the origin of the extreme diversification and abundance of inoceramid bivalves in the Late Cretaceous of the Northwest Pacific. The beginning of inoceramid diversification in the Late Cretaceous is seen during the latest Early Cretaceous, which suggests that M. ipuanus is an important species for inferring the causes of the Late Cretaceous flourishing of inoceramids.