Clitopaxillus fibulatus is an agaric fungus in the family Entolomataceae, a genus closely related to both Clitocybe and Paxillus in broader morphology. The species epithet fibulatus references the presence of clamp connections (fibulae) on hyphae of the fruiting body, a microscopic character of taxonomic significance in distinguishing fungal species and confirming phylogenetic placement. It occurs in temperate European forests and woodland habitats, fruiting in autumn in association with leaf litter and soil organic matter as a saprotrophic or possibly mycorrhizal fungus. The fruiting body is broadly Clitocybe-like in form, with a depressed cap, decurrent gills, and a robust stipe. Molecular phylogenetics has clarified the placement of Clitopaxillus and related genera, confirming their position within Entolomataceae rather than Tricholomataceae despite superficial morphological similarities. C. fibulatus represents the diversity of entolomatoid fungi in European forests, a group of considerable ecological importance in temperate woodland ecosystems contributing to both decomposition and soil food web complexity.