A leaf spot, found on Sophora tonkinensis in Hechi city, Guangxi province, China was identified as Colletotrichum siamense based on morphological and molecular phylogenetic analysis of the internal transcribed spacer (ITS) of ribosomal DNA, β-tublin (TUB2), the translation elongation factor 1-alpha (TEF1-α), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), actin (ACT), chitin synthase 1 (CHS-1), calmodulin (CAL) and glutamine synthetase (GS) genes. Koch's postulates were satisfied by successful reisolation of C. siamense only from plants inoculated with the pathogen. This is the first report of leaf spot caused by C. siamense on Sophora tonkinensis.