Herpothallon inopinatum Frisch & G. Thor sp. nova (Arthoniaceae) is described from Chiapas, Mexico. It is the second species of Herpothallon known with mature ascospores besides H. fertile. It differs from H. fertile in smaller asci immersed in the thallus instead of strongly protruding barrel-shaped thallus warts, smaller bean-shaped ascospores and a thallus chemistry of confluentic and chiodectonic acids. Herpothallon inopinatum most closely resembles H. pustulatum, but differs in the absence of pustules. The closely related H. rubrocinctum has distinct bright-red pseudoisidia and a bright-red pigmentation of pro- and hypothallus, while H. inopinatum lacks pseudoisidia and has a pro- and hypothallus white mottled pale yellow to orange to purplish brown. A revised key to all 34 currently accepted species in Herpothallon is provided.