Chinese chestnut ( Castanea mollissima ) is an important crop tree species in China. However, branch canker and fruit rot are two kinds of severe diseases, which weaken the host and decrease chestnut production. During our investigations into chestnut diseases in China, several fungi have been confirmed as casual agents in previous studies, namely Aurantiosacculus castaneae , Cryphonectria neoparasitica , Cry. parasitica , Endothia chinensis and Gnomoniopsis daii . In this study, a new canker pathogen is introduced based on morphology, phylogeny and pathogenicity. Typical Gnomoniopsis canker sign of wide, orange tendrils emerging from hosts’ glaucous lenticels were obvious on the diseased trees in the field. Symptomatic branches or bark on stems from different chestnut plantations were sampled and isolated, then strains were identified by comparisons of DNA sequence data for the nuclear ribosomal internal transcribed spacer (ITS), partial translation elongation factor-1α ( tef1) and β-tubulin ( tub2) gene regions as well as morphological features. As a result, these strains appeared different from any known Gnomoniopsis species. Hence, we propose a novel species named Gnomoniopsis chinensis . Pathogenicity was further tested using the ex-type strain (CFCC 52286) and another strain (CFCC 52288) on both detached branches and 3-year-old chestnut seedlings. The inoculation results showed that Gnomoniopsis chinensis is mildly pathogenic to Chinese chestnut. However, further studies are required to confirm its pathogenicity to the other cultivated Castanea species in America, Europe and Japan.