Timmia is a small moss genus with only eight currently accepted species. However, the boundaries and taxonomic status of some taxa remain uncertain and controversial, which has significantly hindered studies on its species diversity. During our 2025 expeditions to the Sanjiangyuan Region in Qinghai Province, China, we discovered a moss resembling T. sphaerocarpa with globose capsules, distinguishable by several morphological characters: stomata confined to the capsule base; vegetative and perichaetial leaves with a distinctly differentiated sheathing base; limb laminal cells with distinct mamillae; naked antheridia; sinuose exothecial cells; and spores 2–3 times larger than those of T. sphaerocarpa. Morphological and molecular phylogenetic analyses, based on two chloroplast markers (atpB-rbcL spacer and trnL-trnF) and the nuclear 26S region, suggest that this moss represents a new species, herein described as T. sanjiangyuanensis. A new section, sect. Pseudosphaerocarpa, is established to accommodate this species. Descriptions, illustrations and SEM photographs of the peristome and spores of this new species, along with an updated key to Timmia species in China, are provided.