The cosmopolitan fungal genus Ganoderma is an important pathogen on arboreal plant hosts, particularly in tropical and temperate regions. It has long been used as a traditional medicine because of its medicinal properties and chemical constituents. In this study, Ganoderma collections were made in the Greater Mekong Subregion (GMS), encompassing tropical parts of Laos, Myanmar, Thailand, Vietnam, and temperate areas in Yunnan Province, China. The specimens used in this study are described based on micro-macro-characteristics and phylogenetic analysis of combined ITS, LSU, TEF1α, and RPB2 sequence data. In this comprehensive study, we report 22 Ganoderma species from the GMS, namely, G. adspersum, G. applanatum, G. australe, G. calidophilum, G. ellipsoideum, G. flexipes, G. gibbosum, G. heohnelianum, G. hochiminhense, G. leucocontextum, G. lucidum, G. multiplicatum, G. multipileum, G. myanmarense, G. orbiforme, G. philippii, G. resinaceum, G. sichuanense, G. sinense, G. subresinosum, G. williamsianum, and G. tsugae. Some of these species were reported in more than one country within the GMS. Of these 22 species, 12 were collected from Yunnan Province, China; three were collected from Laos; three species, two new records, and one new species were collected from Myanmar; 15 species and four new records were collected from Thailand, and one new species was collected from Vietnam. Comprehensive descriptions, color photographs of macro- and micro-characteristics, the distribution of Ganoderma within the GMS, as well as a phylogenetic tree showing the placement of all reported Ganoderma from the GMS are provided.