Performance by an Actress in a Leading Role is one of the Academy Awards of merit presented annually by the Academy of Motion Picture Arts and Sciences (AMPAS) to recognize an actress who has delivered an outstanding performance while working within the film industry. Prior to the 49th Academy Awards ceremony (1976), this award was simply known as the Academy Award of Merit for Performance by an Actress. Since its inception, however, the award has commonly been referred to as the Oscar for Best Actress. While actresses are nominated for this award by Academy members who are actors and actresses themselves, winners are selected by the Academy membership as a whole.

Throughout the past 84 years, accounting for ties and repeat winners, AMPAS has presented a total of 85 Best Actress awards to 70 different actresses. Winners of this Academy Award of Merit receive the familiar Oscar statuette, depicting a gold-plated knight holding a crusader's sword and standing on a reel of film. The first recipient was Janet Gaynor, who was honored at the 1st Academy Awards ceremony (1929) for her performances in Seventh Heaven, Street Angel, and Sunrise. The most recent recipient was Meryl Streep, who was honored at the 84th Academy Awards ceremony (2012) for her performance in The Iron Lady.

Best Actress is the name of an award which is presented by various film, television and theatre organizations, festivals, and people's awards. It may refer to:

An Academy Award is an award bestowed by the American Academy of Motion Picture Arts and Sciences (AMPAS) to recognize excellence of professionals in the film industry, including directors, actors and writers. The Oscar statuette is officially named the Academy Award of Merit and is one of nine types of Academy Awards.

The formal ceremony at which the Awards of Merit are presented is one of the most prominent award ceremonies in the world, and is televised live in more than 100 countries annually. It is also the oldest award ceremony in the media; its equivalents, the Grammy Awards (for music), Emmy Awards (for television), and Tony Awards (for theatre) are modeled after the Academy.

The AMPAS was originally conceived by Metro-Goldwyn-Mayer studio boss Louis B. Mayer as a professional honorary organization to help improve the film industry’s image and help mediate labor disputes. The Oscar itself was later initiated by the Academy as an award "of merit for distinctive achievement" in the industry.