Four new species of Begonia (Begoniaceae) are described from the Indonesian island of Sulawesi. All four species ( Begonia chiasmogyna M.Hughes, Begonia macintyreana M.Hughes, Begonia mendumae M.Hughes and Begonia stevei M.Hughes) belong to Begonia section Petermannia, and they increase the number of Begonia species known from the island to 30.