Begonia bangsamoro, a new Philippine Begonia species from the section Petermannia is described and illustrated. The new species was discovered in the fragmented riparian forest of Lanao del Sur, along the Ginapukan river in Wao, Mindanao island, Philippines. The previous collections also revealed that it occurs in the nearby province of Bukidnon. Begonia bangsamoro is distinguished from other Philippine Begonia by its lacerate leaf margins, terminal inflorescences, 4-tepaled staminate flowers, and sparsely hirsute ovaries. The new species is compared with the phenetically similar B. quercifolia. Based on IUCN Red List criteria, B. bangsamoro is designated as Endangered (EN).