Abstract microfungi in the Gnomoniaceae (Diaporthales, Ascomycetes) comprise species commonly reported as pathogens and endophytes on trees and herbaceous hosts primarily from temperate forests of North America, Europe, and Japan. The diversity of Gnomoniaceae in China is poorly known, although several plant families that occur there specifically the Betulaceae are considered important hosts. An exploratory trip to Yunnan, China, resulted in the discovery of several members of the Gnomoniaceae. In this paper a new monotypic genus, Occultocarpon and its species, O. ailaoshanense, are described and illustrated. A phylogeny based on three genes (LSU, rpb2, tef1-α) reveals that O. ailaoshanense belongs to the Gnomoniaceae and forms a branch distinct from the currently known genera. Occultocarpon ailaoshanense is characterized by perithecia with thin, central to eccentric necks in groups embedded in a stroma and oblong elliptical-elongated, one-septate ascospores. Occultocarpon ailaoshanense occurs on the bark of branches of Alnus nepalensis (Betulaceae) in Yunnan, China.