Species of Diaporthe are important phytopathogens with wide host ranges and a global distribution. In the present study multi-locus phylogeny based on combined sequences of rDNA ITS, and partial sequences from the translation elongation factor 1-α (EF 1-α), β tubulin (TUB) and calmodulin (CAL) genes, reveal three new species from fresh collections made in northern Thailand. The new species Diaporthe siamensis, D. thunbergii and D. pterocarpicola are introduced in this paper with full descriptions and comparison with similar taxa. Phomopsis pterocarpi which is epitypified and synonymised under Diaporthe pterocarpi is described based on a collection from northern Thailand.