An account with keys, illustrations, and distribution maps of the three species of North American Aconogonon (= Polygonum sect. Aconogonon) is given. Epidermal anatomy of the leaf and pollen morphology are also briefly studied. One new variety, A. phytolaccifolium var. glabrum S.-P. Hong, is described. Two new combinations proposed are A. hultenianum var. lapathifolium (Cham. & Schlechtend.) S.-P. Hong and A. davisiae var. glabrum (Jones) S.-P. Hong. Two species names are relegated to synonymy. A neotype for A. hultenianum var. lapathifolium (Cham. & Schlechtend.) S.-P. Hong, and a lectotype for A. davisiae (Brewer ex A. Gray) Sojak are selected.