Three species of lichens are described as new from the Chapada dos Guimarães in Mato Grosso (Brazil): Astrothelium muriconicum Aptroot & M.F.Souza, sp. nov. which is the first species in the core group of the genus, the A. conicum-group, with muriform ascospores; Gassicurtia lopesiana M.F.Souza & Aptroot, sp. nov. which is characterized by red apothecium margins and lichexanthone in the thallus; and G. pruinosa M.F.Souza & Aptroot, sp. nov. which has brown pruinose, UV+ green discs and less red pigmantation. A further 191 species are reported new to the state Mato Grosso, 14 of which are first reports from Brazil.