Waltheria belizensis J. G. Saunders, a new distinctive Sterculiaceae species known only from Mountain Pine Ridge National Forest Reserve, Cayo District, Belize, and potentially distylous by floral morphology, is described and illustrated. It is placed in section Waltheria L. in the W. cinerescens A. St.-Hilaire alliance by its penicillate stigma and loculicidal capsule with an extensive corneous endocarp. It is closest to the distylous W. petiolata K. Schumann in Martius from Brazil (Rio de Janeiro) by the decomposite inflorescences; the shape, venation, and vestiture of calyces; and the shapes of petal apices and seeds. Waltheria belizensis differs from W. petiolata by leaves with minute, appressed vestiture, revolute teeth margins, and a greater number of lateral veins; corymbiform inflorescences; and a seed less lobed and less pusticulate. The contrasting characters are detailed in a key. Thrum-like pollen of W. belizensis and thrum pollen of confirmed W. petiolata are reported for the first time. These data suggest the thrum-like pollen of the Belizean species is less derived than that of the Brazilian one. A key is provided to separate W. belizensis from the only other species of Waltheria in Belize, W. indica L. s.l. The present and additional studies concur with previous exclusion of W. glomerata C. Presl from the flora of Belize, based solely on Dwyer & Liesner 12292, the type of W. belizensis. Other Belizean plant endemics found in the Mountain Pine Ridge National Forest Reserve are listed, and the region is discussed.