A new pyrenomycete, Monosporascus ibericus sp. nov., isolated as an endophyte from roots and stems of three plant species growing on sand flats and salt marshes in the Ebro Delta (Spain), is described. The main characteristic of this fungus is the presence of a higher number of ascospores per ascus (up to six), compared to the other species of the genus: M. cannonballus, M. eutypoides, and M. monosporus. M. ibericus produces a cleistothecial ascomata with a tomentose peridium and lacks an anamorph. In addition to the morphological data, the comparative analysis of the ITS-region sequences of M. ibericus and the other Monosporascus spp., has supported the recognition of the new species. A phylogenetic study based on the sequences of the 18S rDNA did not allow us to assess clearly the taxonomic position of the genus Monosporascus, although the results indicated the genus might have affinities to the Xylariales rather than to the Sordariales.