Xiphophorus is a genus of euryhaline and freshwater fishes in the family Poeciliidae of order Cyprinodontiformes. The many Xiphophorus species are all called either platyfish (or platies) or swordtails. The type species is X. hellerii, the Green swordtail. Platyfish and swordtails are Live-bearers, meaning that they reproduce via internal fertilization. Three species and their hybrids are common in the aquarium trade: the Green swordtail, the Southern platyfish (X. maculatus) and the Variable platyfish (X. variatus).
IUCN lists two Xiphophorus species, the Marbled swordtail (X. meyeri) and the Northern platyfish (X. gordoni), as Endangered, while the Monterrey platyfish (X. couchianus) is listed as Critically Endangered.
Xiphophorus species are regularly used in genetic studies and scientists have developed many interspecific hybrids. The Xiphophorus Genetic stock center, founded by Dr Myron Gordon in 1939 is an important source of these fish for research.
In addition, several species are commonly kept by aquarium hobbyists, especially X. helleri, X. maculatus and X. variatus. In fact, these are a key category in fishkeeping, a group of extremely hardy livebearing fish, along with the molly and guppy, that adapt to almost any water conditions, from cold to tropical, freshwater to fully marine. Unlike many species, these are available almost completely as captive-raised, because of the ease with which they breed.
There are currently 26 recognized species in this genus: 
- Xiphophorus alvarezi D. E. Rosen, 1960 (Chiapas swordtail)
- Xiphophorus andersi M. K. Meyer & Schartl, 1980 (Spiketail platyfish)
- Xiphophorus birchmanni Lechner & Radda, 1987 (Sheepshead swordtail)
- Xiphophorus clemenciae Álvarez, 1959 (Yellow swordtail)
- Xiphophorus continens Rauchengerger, Kallman & Morizot, 1990 (Short-sword platyfish)
- Xiphophorus cortezi D. E. Rosen, 1960 (Delicate swordtail)
- Xiphophorus couchianus (Girard, 1859) (Monterrey platyfish)
- Xiphophorus evelynae D. E. Rosen, 1960 (Puebla platyfish)
- Xiphophorus gordoni R. R. Miller & W. L. Minckley, 1963 (Northern platyfish)
- Xiphophorus hellerii Heckel, 1848 (Green swordtail)
- Xiphophorus kallmani M. K. Meyer & Schartl, 2003
- Xiphophorus maculatus (Günther, 1866) (Southern platyfish)
- Xiphophorus malinche Rauchengerger, Kallman & Morizot, 1990 (Highland swordtail)
- Xiphophorus mayae M. K. Meyer & Schartl, 2002
- Xiphophorus meyeri Schartl & Schröder, 1988 (Marbled swordtail)
- Xiphophorus milleri D. E. Rosen, 1960 (Catemaco platyfish)
- Xiphophorus mixei Kallman, Walter, Morizot & Kazianis, 2004 (Mixe swordtail)
- Xiphophorus montezumae D. S. Jordan & Snyder, 1899 (Montezuma swordtail)
- Xiphophorus monticolus Kallman, Walter, Morizot & Kazianis, 2004 (Southern mountain swordtail)
- Xiphophorus multilineatus Rauchengerger, Kallman & Morizot, 1990
- Xiphophorus nezahualcoyotl Rauchengerger, Kallman & Morizot, 1990 (Mountain swordtail)
- Xiphophorus nigrensis D. E. Rosen, 1960 (Panuco swordtail)
- Xiphophorus pygmaeus C. L. Hubbs & Gordon, 1943 (Pygmy swordtail)
- Xiphophorus signum D. E. Rosen & Kallman, 1969
- Xiphophorus variatus (Meek, 1904) (Variable Platyfish)
- Xiphophorus xiphidium (Gordon, 1932) (Swordtail platyfish)