Helicascus species from submerged wood in aquatic habitats in northern Thailand were studied based on morphology and analysis of DNA sequence data. Descriptions and illustrations of H. elaterascus and a new species H. chiangraiensis are provided. H. chiangraiensis is characterized by unilocular ascostromata, fissitunicate asci with a long, narrow and coiled endoascus, ascospores with 2–4 large refractive guttules. The phylogenetic analysis based on combined ITS, LSU and SSU sequence data placed the species in Morosphaeriaceae (Pleosporales).