What films has joaquin phoenix won an academy award for?

Quick Answer
Joaquin Phoenix won his Academy Award for Best Actor for his performance in "Joker" (2019).
Explanation
Joaquin Phoenix received the Oscar for Best Actor at the 92nd Academy Awards for portraying Arthur Fleck in the 2019 film "Joker." The award recognized his transformative, intense performance that anchored the controversial, character‑driven origin story. While Phoenix has earned numerous nominations and other major awards for films such as "Gladiator," "Walk the Line," and "The Master," "Joker" remains the only movie for which he has actually won an Academy Award. His win marked a significant milestone in his career, adding the Academy’s top acting honor to his previous accolades, including a Golden Globe and a BAFTA.
The Academy’s recognition highlighted Phoenix’s ability to convey deep psychological complexity, a trait that has defined much of his work. The film’s success at the ceremony also reflected the broader critical and commercial impact of "Joker," which sparked extensive discussion about its themes and tone. Phoenix’s Oscar win solidified his status as one of the most distinguished actors of his generation.
No other film in his filmography has resulted in an Academy Award win to date.