Koenigia arunachalensis sp. nov., a new species of Polygonaceae, from Tawang district of Arunachal Pradesh, India is described and illustrated. A detailed description, illustration, color photoplate with photographs of habit and habitat, and a distribution map are provided. The new species closely resembles K. nepalensis but can be distinguished by its lanceolate leaves (versus ovate), bipartite ochrea (versus funnel-shaped), lanceolate bract (versus elliptic-ovate), inflorescence with 8-25 flowers (versus 5-10 flowers), pedicel 1.8-2.2 mm long (versus 1-1.5 mm), stamens 5 (versus 3) and staminodes absent (versus present, 5). It is highly abundant in the study areas and was observed from the Indo-Bhutan border to the Indo-Tibet border. Provisionally, the new species has been assigned as Data Deficient (DD) following IUCN guidelines.