A new species Ganoderma carocalcareus (Basidiomycota, Ganodermataceae) was collected on living trunk and dead stumps of Anthocleista nobilis (Gentianaceae) in waterlogged swamps in the Mbalmayo Forest Reserve, Cameroon, and identified on the basis of morphology and phylogenetic analyses inferred from mitochondrial small subunit (mtSSU) and internal transcribed spacer (ITS1-5.8S-ITS2) rDNA sequences. Distinct phenotypic characteristics of the new species are dimorphism of basidiomata and variability in context structure and texture over developmental stages. The young basidiomata is ungulate to punk-shaped with context composed of vegetative hyphae attended by scattered, orbicular, smooth, thick-walled chlamydospores, and the mature basidiomata is cushion- to bracket-like with context entirely consisting of chlamydospores masses. This ontogeny intimates the origin of chlamydospores, for which the biogenesis correlates the vanishing of vegetative hyphae throughout the basidiomata maturation. Morphological comparison included Tomophagus colossus (=G. colossus), G. subamboinense and G. weberianum, the known Ganodermataceae species producing chlamydospores and or gasterospores in basidiomata tissues, and G. resinaceum, the closest species with regard to morphology. It followed that G. carocalcareus could not be assigned to these or any other known Ganoderma species. Analyses of mtSSU and ITS rDNA sequence data resolved G. carocalcareus in the G. resinaceum group as a distinct species, but being a close relative of both G. subamboinense and G. weberianum.