In Mesoamerican folk religion, a Nagual or Nahual (both pronounced [na'wal]) is a human being who has the power to magically turn him- or herself into an animal form: most commonly a donkey, turkey, or dog,but also other and more powerful animals such as the jaguar and puma.

Such a Nagual is believed to use his powers for good or evil according to his personality. Specific beliefs vary, but the general concept of nagualism is pan-Mesoamerican. Nagualism is linked with pre-Columbian shamanistic practices through Preclassic Olmec depictions which are interpreted as humans transforming themselves into animals. The system is linked with the Mesoamerican calendrical system, used for divination rituals.

The birth date often determines if a person will be a Nagual. Mesoamerican belief in tonalism, wherein all humans have an animal counterpart to which their life force is linked, is also part of the definition of nagualism. In English the word is often translated as "transforming witch", but translations without the negative connotations of the word witch would be "transforming trickster" or "shape shifter".

The word nagual derives from the Nahuatl word nahuālli [naˈwaːlːi], an indigenous religious practitioner, identified by the Spanish as a 'magician'. The nagual is acquired along with the other characteristics of a person's birth day at birth. Each day is associated with an animal which has strong and weak aspects. A person born on "The Dog Day" would have both strong and weak 'Dog' aspects.

In Nahuatl the word tonalli was used to refer both to a day and to the animal associated with that day. The nagual is different, where the tonal is the day spirit proper, the nagual is the spirit familiar of the day. It is probable that the tonal represents the daytime aspect and the nagual the nighttime aspect of the tonalli, 'the things of the day'. Because practitioners of powerful magic were normally born on certain days related to animals with a strong or harmful aspect they would often have specific tonals such as the jaguar or puma. In Aztec mythology the God Tezcatlipoca was the protector of nagualism, because his tonal was the jaguar and he governed the distribution of wealth.