A new species of Kirschsteiniothelia from dead branches of sessile oak, K. arasbaranica, is described from Arasbaran forests of Iran, using morphological and molecular data. Kirschsteiniothelia arasbaranica, lacking an anamorph, is characterized by possessing eight-spored, cylindrical-clavate, bitunicate, fissitunicate asci, 120-180 × 30-40 µm, with an apical ocular chamber 5–8 µm wide, and ascospores narrowly to broadly ellipsoidal, 1-septate, verrucose to finely spinulose, (30-)34-42(-44) × (12-)13-16(-18) µm, with a distinct mucilaginous sheath. Phylogenetic analysis of a combined sequence dataset of the ITS, LSU and SSU rDNA also supports the recognition of this fungus as a new species. We hereby propose an emendation of the family Kirschsteiniotheliaceae. A dichotomous key for identification of 20 sufficiently documented Kirschsteiniothelia species is also provided.