A monograph of Castelnavia is presented. Phylogenetic analyses of morphological characters reveal eight nonhomoplasious synapomorphies supporting the monophyly of the genus: 1) absence of roots, a 2) unilocular mature, 3) anisolobous ovary that is 4) surrounded by stem tissue during and after anthesis, 5) horizontal at anthesis, with an 6) asymmetrically inflated pedicel apex and 7) longitudinal axis at 45–90° angle relative to pedicel axis, and 8) one deciduous capsule valve. Five species and two forms are recognized: Castelnavia fluitans, C. monandra, C. multipartita (C. m. forma multipartita, C. m. forma pendulosa C. T. Philbrick & C. P. Bove), C. noveloi, and C. princeps. Four species accepted by earlier authors are placed in synonymy. The morphology and ecology of the genus is discussed, and species descriptions, illustrations, a distribution map, lists of specimens examined, and a key to species are presented. Castelnavia occurs primarily in Brazil with one species in Bolivia. The greatest species diversity is in the Brazilian states of Mato Grosso, Pará, and Tocantins.