The Mexican Jay, Aphelocoma ultramarina, formerly known as the Gray-breasted Jay, is a New World scrub-jay native to the Sierra Madre Oriental, Sierra Madre Occidental, and Transvolcanic Belt of Mexico. It reaches north to southeast Arizona, southwest New Mexico and westernmost Texas in the United States. Its preferred habitat is montane pine-oak forest.

It is a medium-large passerine similar in size to most other jays, with a blue head, blue-gray mantle, blue wings and tail, and pale gray breast and underparts. The sexes are morphologically identical, and juveniles differ only in having less blue coloration and, in some populations, a yellow (instead of black) bill that progressively becomes more black with age. The iris is brown and legs are black. It is most readily distinguished from the similar Western Scrub-jay by the plain (unstreaked) throat and breast, and the mantle contrasting less with the head and wings. Its range somewhat overlaps with the Western Scrub-Jays, but, where they co-occur, the two species seem to show ecological and morphological character displacement (Curry et al. 2002).

In the winter, the Mexican Jay's diet consists mainly of acorns and pine nuts, which are stored in the autumn. However, they are ominvorous in all seasons and their diet includes a wide variety of plant and animal matter, including invertebrates, small amphibians and reptiles, and birds' eggs and nestlings.

It has a cooperative breeding system similar to that of the related Florida Scrub-Jay, with several birds helping at a nest; these 'helpers' are usually immature offspring of the dominant pair from the previous 1-2 years, but also include apparently unrelated flock members.