Who was the first person to be nominated posthumously for the Best Actor Oscar?

In recognition of his performance in East of Eden, Dean was nominated posthumously for the 1956 Academy Awards as Best Actor in a Leading Role of 1955, the first official posthumous acting nomination in Academy Awards history.

James Byron Dean (February 8, 1931 – September 30, 1955) was an American actor. He is remembered as a cultural icon of teenage disillusionment and social estrangement, as expressed in the title of his most celebrated film, Rebel Without a Cause (1955), in which he starred as troubled teenager Jim Stark. The other two roles that defined his stardom were loner Cal Trask in East of Eden (1955) and surly ranch hand Jett Rink in Giant (1956).

Dean's premature death in a car crash cemented his legendary status. He became the first actor to receive a posthumous Academy Award nomination for Best Actor, and remains the only actor to have had two posthumous acting nominations. In 1999, the American Film Institute ranked him the 18th best male movie star of Golden Age Hollywood in AFI's 100 Years...100 Stars list.