Cuphea fluviatilis S. A. Graham, a new species of the Lythraceae from the margins of the Río Samaná in Antioquia Department, Colombia, is described and illustrated. The taxonomic relationships of the species are unsettled. The species bears flowers closely similar to those of species in section Amazoniana (Lourteig) Lourteig, but lacks the distinctive malpighiaceous trichomes characterizing the section; its pollen morphology, a significant indicator of broad relationships in Cuphea, is also unlike that of the section. Pollen of section Amazoniana is typically nonsyncolpate, without protruding pores, and with a psilate to finely rugulate exine; that of C. fluviatilis is syncolpate with protruding pores and a striate exine, a type common to numerous species currently classified in the polyphyletic sections Brachyandra Koehne and Euandra Koehne.