Size and shape profoundly inﬂuence an organism’s ecophysiological performance and evolutionaryﬁtness, suggesting a link between morphology and diversity. However, not much is known abouthow body shape is related to taxonomic richness, especially in microbes. Here we analyse globaldatasets of unicellular marine phytoplankton, a major group of primary producers with an excep-tional diversity of cell sizes and shapes and, additionally, heterotrophic protists. Using two mea-sures of cell shape elongation, we quantify taxonomic diversity as a function of cell size andshape. We ﬁnd that cells of intermediate volume have the greatest shape variation, from oblate toextremely elongated forms, while small and large cells are mostly compact (e.g. spherical or cubic).Taxonomic diversity is strongly related to cell elongation and cell volume, together explaining upto 92% of total variance. Taxonomic diversity decays exponentially with cell elongation and dis-plays a log-normal dependence on cell volume, peaking for intermediate-volume cells with com-pact shapes. These previously unreported broad patterns in phytoplankton diversity revealselective pressures and ecophysiological constraints on the geometry of phytoplankton cells whichmay improve our understanding of marine ecology and the evolutionary rules of life.