Poaceascoma aquaticum sp. nov. (Lentitheciaceae) was found on decaying bamboo submerged in a freshwater stream in northern Thailand. The new species is introduced based on its unique morphology and phylogeny. Poaceascoma aquaticum is characterized by black, semi-immersed or erumpent, papillate ascomata, bitunicate, cylindrical to cylindric-clavate, short pedicellate asci and filiform, pale brown to brown ascospores. Phylogenetic analyses of combined LSU, SSU and RPB2 sequence data shows that Poaceascoma aquaticum forms a robust clade with P. helicoides in the family Lentitheciaceae (Pleosporales). Poaceascoma aquaticum is described, illustrated and compared with related taxa.