Heteranthera longirachilla is a new species described from collections made since the 1960s that were mainly determined as H. reniformis. It is commonly found in streams and waterbodies in the Atlantic Forest and Recôncavo Basin of Bahia, Brazil, and can be distinguished by its long inflorescence rachis that surpasses the spathe apex, and glabrous (rarely with some sparse, short trichomes) filament of the longest stamen. The new species is described and illustrated. A distribution map of the Heteranthera reniformis complex in Brazil and an identification key to all Neotropical Heteranthera species are also provided.