Leiodontium geminoserratum S. He & Chantanaorr. (Hypnaceae), a new species from Thailand, is described herein based on distinctive morphological features. The new species resembles L. gracile Broth. from China and Nepal, but differs most notably in having moderately complanate leafy stems and branches, prorate laminal cells, doubly serrate basal leaf margin, and axillary hairs with a globose distal cell. A full description and photographs of the new species are provided.