Taxonomy of the Postia caesia complex is revised based on morphology and two genetic markers, ITS and tef1. In total, we recognize 24 species, multiplying the known species diversity in the complex. We provide descriptions for 20 temperate Northern Hemisphere taxa. Identity of the core species, P. caesia, is re-established, and a neotype from the type locality is selected. Four new combinations are proposed, and 10 new species are described: P. arbuti, P. auricoma, P. bifaria, P. comata, P. cyanescens, P. glauca, P. livens, P. magna, P. populi, and P. yanae.