Coccomyces tumidus is a small discomycete fungus in the family Rhytismataceae, found decomposing fallen leaves and plant debris in temperate European woodlands. The genus Coccomyces belongs to the order Rhytismatales, a group of ascomycetes that predominantly occupy a saprotrophic niche on dead plant material, though some relatives in the family are pathogenic on living plant hosts. Coccomyces tumidus produces the characteristic small, dark, disc-shaped apothecia typical of the genus, which emerge from or are embedded in the surface of decomposing leaves. The species epithet 'tumidus'—Latin for swollen—likely refers to a distinctive swollen or raised morphology of the stromata or fruiting bodies. This species has been recorded from European forest habitats, where it colonizes the leaf litter layer of deciduous forests, assisting in the physical and chemical breakdown of plant debris. The fruiting bodies produce asci containing ascospores that are dispersed by air currents to infect new substrate during periods of wet weather conducive to spore germination. Coccomyces species as a group present taxonomic challenges due to the small size and morphological similarity of fruiting bodies, and molecular phylogenetic methods have been necessary to clarify species boundaries within the genus. As a decomposer, C. tumidus contributes to carbon and nutrient cycling in European forest ecosystems. Its conservation status has not been assessed by the IUCN.