The Big Bend gambusia (Gambusia gaigei) is a rare species of fish in the family Poeciliidae.

[4] It is endemic to the Big Bend region of the Rio Grande of the United States and Mexico.

The only known remaining population is in a protected pond in the Big Bend National Park.

[5] This livebearer is about 2 inches (5.1 cm) long.

[5] This species was described by the American ichthyologist Carl Leavitt Hubbs in 1929 from a type collected in slough close to the Rio Grande at Boquillas, Brewster County, Texas[6] by Frederick McMahon Gaige (1890–1976), a zoologist who was director of the Zoological Museum of the University of Michigan.