Oppenheimer (/ˈɒpənhaɪmər/ OP-ən-hy-mər) is a 2023 epic biographical thriller film written and directed by Christopher Nolan. It stars Cillian Murphy as J. Robert Oppenheimer, the American theoretical physicist credited with being the "father of the atomic bomb" for his role in the Manhattan Project—the World War II undertaking that developed the first nuclear weapons. Based on the 2005 biography American Prometheus by Kai Bird and Martin J. Sherwin, the film chronicles the career of Oppenheimer, with the story predominantly focusing on his studies, his direction of the Manhattan Project during World War II, and his eventual fall from grace due to his 1954 security hearing. The film also stars Emily Blunt as Oppenheimer's wife "Kitty", Matt Damon as head of the Manhattan Project Leslie Groves, Robert Downey Jr. as U.S. Atomic Energy Commission member Lewis Strauss, and Florence Pugh as Oppenheimer's communist lover Jean Tatlock. The ensemble supporting cast includes Josh Hartnett, Casey Affleck, Rami Malek, and Kenneth Branagh.

The film was announced in September 2021 after Universal Pictures won a bidding war for Nolan's screenplay, following Nolan's conflict with longtime distributor Warner Bros. Murphy was the first cast member to sign on the following month, with the rest of the cast joining between November 2021 and April 2022. Pre-production was under way by January 2022, and filming took place from February to May. Oppenheimer was filmed in a combination of IMAX 65 mm and 65 mm large-format film, including, for the first time, scenes in IMAX black-and-white film photography. Like his previous works, Nolan made extensive use of practical effects, with minimal computer-generated imagery used to perfect the former. Editing was handled by Jennifer Lame, and the score was composed by Ludwig Göransson. The film is Nolan's fourth to receive an R-rating in the United States, preceded by Following (1998), Memento (2000) and Insomnia (2002).

Oppenheimer premiered at Le Grand Rex in Paris on July 11, 2023, and was theatrically released in the United States and the United Kingdom on July 21 by Universal. Its simultaneous release with Warner Bros.'s Barbie led to the Barbenheimer cultural phenomenon, which encouraged audiences to see both films as a double feature. The film received critical acclaim and grossed over $950 million worldwide, becoming the third-highest-grossing film of 2023, the highest-grossing World War II-related film, the highest-grossing biographical film, and the second-highest-grossing R-rated film.