﻿AbstractA new species of pelagic diatom,Pleurosigmapacificumsp. nov., is described from the tropical Western Pacific Ocean. It has the typical features ofPleurosigma, including a slightly sigmoid raphe, intersected transverse and oblique striae, and loculate areolae with external opening slits and internal poroids. Morphologically,P.pacificumbelongs to a species group ofPleurosigmawith lanceolate valves, includingP.atlanticumHeiden & Kolbe,P.nubeculaW. Smith,P.indicumSimonsen, andP.simonseniiHasle. However,P.pacificumdiffers by its smaller lanceolate valve and smaller intersection angle as well as elliptical areolae without a silica bar. The SSU rDNA andrbcL sequence data placeP.pacificumin a basal position relative to other species ofPleurosigma. Our molecular phylogenetic analyses did not support the monophyly of lanceolate and slightly sigmoid species. Thus, the sigmoidality of valve outline cannot be considered as a criterion to define the species group.