Once Upon a Time in Mexico is a 2003 American neo-Western action film written, directed, produced, photographed, scored, and edited by Robert Rodriguez. It is the sequel to Desperado (1995) and the third and final installment in the Mexico Trilogy. The film features Antonio Banderas in his second and final performance as El Mariachi. In the film, El Mariachi is recruited by CIA agent Sheldon Sands (Johnny Depp) to kill a corrupt general responsible for the death of his wife, Carolina (Salma Hayek).

Once Upon a Time in Mexico was released in the United States on September 12, 2003, by Sony Pictures Releasing. The film received mixed reviews from critics, with praise for Depp's performance, but criticism for reducing its protagonist to an almost secondary character in his own trilogy and a convoluted plot. In the special features of the film's DVD, Rodriguez explained this was intentional, as he wanted this to be his The Good, the Bad and the Ugly of the trilogy. It grossed over $98 million against a $29 million production budget.