The Monterrey platyfish (Xiphophorus couchianus) is a species of freshwater fish in family Poecilidae of order Cyprinodontiformes. A live-bearer, it is native to the vicinity of Monterrey, Mexico.
IUCN lists the Monterrey platyfish, formerly widespread in Mexico, as Critically Endangered, upgraded from Endangered in 1996. The fish's population, numbering fewer than 250 individuals, is falling rapidly and is confined to a very small area, occurring only in fragmented pockets of habitat populated by few individuals. It is additionally suffering from environmental degradation.
X. couchianus is harvested for the aquarium fish trade. It has also been used for genetic research.
Sexual dimorphism is moderate, the male Monterrey platyfish growing to a maximum overall length of 4.0 cm (1.6 in) and the female 6.0 cm (2.4 in).