Decaisnella formosa sp. nov., an undescribed ascomycete, was discovered on intertidal wood from a beach on the Mornington Peninsula, National Park, Victoria, Australia. Of the eleven species described under the genus Decaisnella, none match D. formosa, which is described here as a new species.Key words: Loculoascomycetes, Melanommatales, marine fungi, muriform ascospores.