Plant litter imported from forest headwater streams is an important supply of material and energy to the downstream aquatic ecosystems, affecting the storages and fluxes of carbon and nutrients of the whole forest ecosystems. However, our knowledge on the dynamics of plant litter potassium (K) storage is still limited. Here, we monitored the spatial and temporal characteristics of plant litter K concentration and storage in a subtropical forest headwater stream for an entire year. Results showed that: (1) the annual average concentration and storage of K in plant litter were 1.4 mg/kg and 19.6 mg/m², respectively, with an overall decreasing trend throughout the year and from the source of the stream to the mouth; (2) riparian forest type (broadleaved vs. mixed forests) and the presence of a tributary showed significant effects on twig K concentration and storage, but did not affect those of other types of plant litter (total, fine woody debris, leaf, reproductive part, and twig); (3) water depth and active channel width positively correlated with litter K storage, while stream gradient, water dissolved oxygen, and alkalinity negatively corrected to litter K storage. Our results clearly demonstrated the spatiotemporal dynamics of plant litter K storage in the subtropical forest headwater stream, which provides basic data and scientific basis for an in-depth understanding of nutrient fluxes and cycling along with ecohydrological processes in subtropical forest ecosystems.