Once Upon a Time in Mexico is a 2003 American action film written, produced, edited, cinematographied, scored, and directed by Robert Rodriguez. It is the third and final film in Rodriguez's Mexico Trilogy, and is a sequel to El Mariachi (1992) and Desperado (1995). 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 Armando Barillo (Willem Dafoe), a Mexican drug lord who is planning a coup d'état against the President of Mexico. At the same time, El Mariachi seeks revenge against a corrupt general responsible for the death of his wife, Carolina (Salma Hayek).

Once Upon a Time in Mexico was not only the first film Rodriguez ever shot in digital HD (instead of 35mm film) but was also one of the first high-budget films shot in HD pre-dating Star Wars Episode II: Attack of the Clones.

Once Upon a Time in Mexico received positive reviews, but was criticized for reducing its protagonist to an almost secondary character in his own trilogy and for having a convoluted plot. In the special features of the film's DVD, Rodriguez has explained that this was intended, as he wanted this to be The Good, the Bad and the Ugly of the trilogy. Once Upon a Time in Mexico holds the box office record for being the most improved second sequel of all-time, grossing 122% more than Desperado.