Rivulus is a genus of small freshwater fish in the Cyprinodontiformes family Rivulidae. It was traditionally considered to be the largest genus in its family; however, the genus's size is currently in dispute. Wilson J. E. Costa split this genus into several new genera (including Anablepsoides, Atlantirivulus, Cynodonichthys, Kryptolebias, Laimosemion and Melanorivulus) in 2004 and 2011, leaving only a few Greater Antillean species in Rivulus itself. Despite being moved to other genera, some of the species retain the common name "rivulus", like the well-known mangrove rivulus (Kryptolebias marmoratus). Shortly after the review by Costa, another review authored by J.H. Huber refuted the split, moving the proposed genera back in Rivulus and again making the genus the largest in the family Aplocheilidae.

If the split is recognized, the genus Rivulus only includes three non-annual killifish species from western Cuba (including Isla de la Juventud) and Hispaniola where they inhabit streams, rivers and lagoons from the highlands to the coast. If the split into multiple genera is not recognized, Rivulus contains more than 150 non-annual killifish species found in fresh and brackish waters of the tropical and subtropical Americas, ranging from Argentina in south to Mexico in north, including the Caribbean region.

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