The 12th Academy Awards ceremony, held on February 29, 1940, by the Academy of Motion Picture Arts and Sciences (AMPAS), honored the best in film for 1939 at a banquet in the Coconut Grove at The Ambassador Hotel in Los Angeles. It was hosted by Bob Hope, in his first of nineteen turns as host.

David O. Selznick's Gone with the Wind received the most nominations of the year with thirteen, winning eight Oscars (both records at the time). It became the first film in color that won Best Picture. This year was the first in which multiple films received ten or more nominations (Mr. Smith Goes to Washington received eleven).

This was the first year in which Best Visual Effects was a competitive category; previously, "special achievement" awards for effects had occasionally been conferred. This year, Best Cinematography was split into Color and Black & White categories.

Hattie McDaniel became the first African-American to receive an Academy Award, winning Best Supporting Actress for Gone with the Wind. Mickey Rooney became the second-youngest nominee for Best Actor at 19, and the first teenager to be nominated for an Academy Award, for his performance in Babes in Arms.

Nominees were announced on February 11, 1940. AMPAS presented Academy Awards of Merit in 20 categories. Nominees for each award are listed below; award winners are listed first and highlighted in boldface.

The ceremony presenters are listed below in the sequence of awards presented.

Prior to the announcement of nominations, Mr. Smith Goes to Washington and Gone with the Wind were the two films most widely tipped to receive a significant number of nominations. Mr. Smith Goes to Washington premiered in Washington with a premier party hosted by the National Press Club who found themselves portrayed unfavourably in the film; the film's theme of political corruption was condemned and the film was denounced in the U.S. Senate. Joseph P. Kennedy, the U.S. Ambassador to Britain urged President Franklin D. Roosevelt and the studio head Harry Cohn to cease showing the film overseas because "it will cause our allies to view us in an unfavourable light". Among those who campaigned in favour of the film were Hedda Hopper who declared it "as great as Lincoln's Gettysburg speech", while Sheilah Graham called it the "best talking picture ever made". Screen Book magazine stated that it "should win every Academy Award". Frank Capra, the director, and James Stewart, the film's star were considered front runners to win awards.

Gone with the Wind premiered in December 1939 with a Gallup poll taken shortly before its release concluding that 56.5 million people intended to see the film. The New York Film Critics Award was given to Wuthering Heights after thirteen rounds of balloting had left the voters deadlocked between Mr. Smith Goes to Washington and Gone with the Wind. The press were divided in their support for the nominated actors. Time magazine favoured Vivien Leigh and used her portrait for their Christmas 1939 edition, and The Hollywood Reporter predicted a possible win by Leigh and Laurence Olivier with the comment that they "are, for the moment, just about the most sacred of all Hollywood's sacred cows". West Coast newspapers, particularly in Los Angeles, predicted Bette Davis would win for Dark Victory. Observing that Davis had achieved four box office successes during the year, one paper wrote, "Hollywood will stick by its favourite home-town girl, Bette Davis".