Begonia kayinensis, a new species from Kayin State, Myanmar is described and illustrated. This new species is identified as a member of Begonia section Monophyllon A.DC. in its morphological characteristics including the upright stem with tuber, inflorescences arising from the top of the stem, and 2-loculed ovary. A description, photographs, illustration, and comparison with allied species in sect. Monophyllon and provisional assessment of the conservation status are provided.