21 Grams is a 2003 American psychological drama film directed by Alejandro González Iñárritu from a screenplay by Guillermo Arriaga, based on a story by Arriaga and Iñárritu. The film stars Sean Penn, Naomi Watts, Charlotte Gainsbourg, Danny Huston and Benicio Del Toro. The second part of Arriaga's and Iñárritu's "Trilogy of Death", preceded by Amores perros (2000) and followed by Babel (2006), 21 Grams interweaves several plot lines in a nonlinear arrangement.

The film's plot is about the consequences of a tragic hit-and-run accident. Penn plays a critically ill mathematician, Watts plays a grief-stricken mother, and Del Toro plays a born-again Christian ex-convict whose faith is sorely tested in the aftermath of the accident. The three main characters each have "past", "present" and "future" story threads, which are shown as non-linear fragments that punctuate elements of the overall story, all imminently coming toward each other and coalescing as the story progresses. Critics gave the film generally favorable reviews, praising Iñárritu's direction, Arriaga's screenplay, and Watts' and Del Toro's performances. 21 Grams made $60.4 million at the box office on a $20 million budget. Watts and Del Toro were nominated for Best Actress and Best Supporting Actor at the 76th Academy Awards.