Endococcus grumantianus, a pyrenomycete growing on the lichen Steinia geophana as part of the soil crust, next to cyanobacteria, is described as new to science from the arctic tundra of Svalbard. The species is characterized by well-developed periphysoids, and verruculose, mainly heteropolar, relatively large ascospores, (14−)16−21(−27.5) × (6−)7.5−9.5(−11) μm, usually with one, but occasionally with 2−3 septa, which is an exception in this genus. A key for 14 Endococcus species growing on terricolous lichens is provided. Key words: Arctic, biological soil crusts, key, parasites, taxonomy, Verrucariaceae.