Castanopsis pathakii (Fagaceae), a new species is described and illustrated based on the study of specimens collected from Upper Siang district of Arunachal Pradesh, India, housed at CAL. Detailed notes on its diagnostic traits and morphology are provided. Its relationship with the morphologically allied species C. fleuryi Hickel & A.Camus is discussed. The new species is also compared with C. echinocarpa Miq. and C. brevispinula Hickel & A.Camus. © 2019, The Board of Trustees of the Royal Botanic Gardens, Kew.