Conceveiba belizensis McPherson & B. Holst (Euphorbiaceae), a new species of Conceveiba Aubl., is described, differing notably from the similar C. guianensis Aubl. and C. rhytidocarpa Müll. Arg. in having staminate inflorescences unbranched (vs. paniculiform), staminate flowers with pedicels 4–9 mm long (vs. less than 2 mm) and staminodes much shorter than the stamens (vs. staminodes much longer than the stamens), and pistillate flowers sessile (vs. pedicels at least 2 mm long).