Marine diatrypaceous fungi are associated with many host species, including Avicennia spp., Halosarceia halocnemoides (Nees) Paul G.Wilson, Kandelia candel (L.) Druce, Salicornia sp., Rhizophora apiculata Blume and Suaeda fruticosa (L.) Forssk. Accurate morphological identification of individual taxa of this family is difficult, as previous classifications generally lack sequence data. We provide a detailed account of all known marine diatrypaceous taxa and determine their phylogenetic and morphological relationships. A novel genus Halocryptovalsa Dayarathne & K.D.Hyde, gen. nov. is established to accommodate Halocryptovalsa salicorniae Dayarathne & K.D.Hyde, sp. nov., and Halocryptovalsa avicenniae, comb. nov. (synonym of Cryptovalsa avicenniae). Three novel species Cryptosphaeria avicenniae Devadatha & V.V.Sarma, sp. nov., Cryptosphaeria halophila Dayarathne & K.D.Hyde, sp. nov., and Diatrype mangrovei Dayarathne & K.D.Hyde, sp. nov., are introduced from mangroves. Halodiatrype is amended with asexual morph characters.