Aeschynomene fluvialis, a new species from the North region of Brazil, is described and illustrated here and has its morphological relationships, systematic position, geographic distribution, ecology, conservation status, and period of flowering and fruiting provided. It can be differentiated from its congeners, especially A. pratensis and A. montevidensis, by a set of characters such as cespitose habit, venation and margin of leaflets, size of flowers, diadelphous androecium, and loment characters. A key to identify all Aeschynomene species with medifixed stipules, fruit with articles separated by septa, and bilabiate calyx, which are common in swampy or aquatic environments from the North region of Brazil, is also provided.