A new species of Orthotrichum, O. dentatum T.Kiebacher & Luth, is described from the European Alps. The new species belongs to the subgenus Pulchella (Schimp.) Vitt, section Diaphana Vent. Orthotrichum dentatum is similar to O. schimperi Hammar but differs from the latter in that it features a hairy calyptra, denticulate to dentate leaf apices and strongly keeled leaves. Orthotrichum dentatum grows as an epiphyte on a variety of deciduous trees. So far, the new species has only been found in Italy and Switzerland.