A new species of Helicascus, H. mangrovei, on dead intertidal mangrove wood was collected. Morphology of this fungus was studied and DNA sequence analyses of the internal transcribed spacers (ITS), small subunit (SSU) and large subunit (LSU) ribosomal (r) DNA, the translation elongation factor 1-alpha gene (TEF-1α) and the RNA polymerase II gene (RPB2) were used to determine the phylogeny of this fungus. Helicascus mangrovei possesses semi-immersed lenticular ascomata and stroma-like structures covered with a carbonaceous, dark brown to black pseudoclypeus. Its ascomata do not have a separate locule. The bitunicate asci have an ocular chamber and a long stipe. Ascospores are obovoid, 1-septate, uniseriate, unequally two-celled, slightly constricted at the septum, thick-walled, with larger brownish apical cells and smaller basal cells, one apiculate end, hyaline when young, becoming dark brown when mature. Helicascus mangrovei forms a well-supported clade within Helicascus species and is closely related to marine species within the Morosphaeriaceae, Pleosporales. Based on the morphological features and molecular data, a new marine species of Helicascus is proposed.