The axolotl ( ; from Classical Nahuatl: āxōlōtl [aːˈʃoːloːtɬ] ) (Ambystoma mexicanum) is a paedomorphic salamander, one that matures without undergoing metamorphosis into the terrestrial adult form; adults remain fully aquatic with obvious external gills. This trait is somewhat unusual among amphibians, though this trait is not unique to axolotls, and this is apparent as they may be confused with the larval stage or other neotenic adult mole salamanders (Ambystoma spp.), such as the occasionally paedomorphic tiger salamander (A. tigrinum) widespread in North America; or with mudpuppies (Necturus spp.), which bear a superficial resemblance but are from a different family of salamanders.

Axolotls originally inhabited a large lake in the Mexican highlands known as Lake Texcoco, along with a number of smaller, interconnected lakes such as Lake Xochimilco and Lake Chalco, being abundant enough to form a staple in the Aztec diet, being sold as food in the markets of Tenochtitlan. These lakes were mostly drained by Spanish settlers after the conquest of the Aztec Empire, leading to the destruction of much of the axolotl's natural habitat, which is now largely occupied by Mexico City. Due to continued urbanization in Mexico City, which causes water pollution in the remaining waterways, as well as the introduction of invasive species such as tilapia and perch, the axolotl is near extinction, the species being listed as critically endangered in the wild, with a decreasing population of around 50 to 1,000 adult individuals, by the International Union for Conservation of Nature (IUCN) and is listed under Appendix II of the Convention on International Trade in Endangered Species (CITES).

A large captive population of axolotls currently exist, with the specimens being used extensively in scientific research for their remarkable ability to regenerate parts of their body, including limbs, gills and parts of their eyes and brains. In general, they are model organisms that are also used in other research matters, and as aquarium technology developed, they have become a common exhibit in zoos and aquariums, and as an occasional pet in home aquaria. Axolotls are also a popular subject in contemporary culture, inspiring a number of works and characters in media.