Pinguiochrysis pyriformis gen. et sp. nov. is a brown, naked, non-motile, marine picoplankton. A culture was established from a surface sample collected in 1991 from the tropical Western Pacific Ocean. Typical cells of P. pyriformis are distinctively pear-shaped and have one ovoid chloroplast; these two features distinguish this species from the other picophytoplankton species. However, the pyriform morphology is not consistent and cells frequently change to a subspherical shape. The chloroplast and mitochondrion ultrastructure confirm that this species belongs to the photosynthetic stramenopiles (chromophytes). Additional distinctive ultrastructural characteristics of P. pyriformis include (i) a chloroplast envelope forming a tubular invagination that penetrates into the pyrenoid; (ii) thylakoid lamellae consisting of more than three layers in some cells; (iii) the lack of basal bodies and centrioles; and (iv) the lack of scales or other extracellular structures. Based on the morphological features, this picoplanktonic species was described as a new species and placed in the Pinguiophyceae on the basis of the molecular phylogenetic analysis and biochemical data published elsewhere.