Orthotrichum cambrense Bosanquet et F. Lara, is described from material recently collected on trees in a small area of Carmarthenshire, south Wales. The new moss has a unique combination of morphological traits, several of them highly distinctive: characteristic almost flat leaves, with plane margins and frequently rounded apices; brood bodies common on leaves and rhizoids; emergent furrowed capsules with cryptopore stomata in their lower third; a peristome of 8 pairs of teeth and 8 segments; and the calyptra hairy, especially in the upper part. This moss is not believed to be close to other species with plane leaf-margins in the subgenus Pulchella. However, several gametophytic and sporophytic details relate Orthotrichum cambrense to O. tenellum and similar species in this same subgenus. Reasons why such a distinctive moss has gone unnoticed until now are discussed.