Crowing a three-decade career, the 46-year-old blonde with the icy blue eyes took one of Hollywood's top prizes for playing a single mother in director Richard Linklater's coming-of-age drama "Boyhood."
After thanking her family and co-stars, Arquette made an impassioned plea for "wage equality once and for all, and equal rights for women in the United States of America."
The Illinois native had long been the favorite in the category after winning a Golden Globe, a BAFTA and other accolades in the run-up to the Oscars.
She defeated Emma Stone ("Birdman"), Laura Dern ("Wild"), three-time winner Meryl Streep ("Into the Woods") and Britain's Keira Knightley ("The Imitation Game").
The role of Olivia in "Boyhood" -- she chose the name in tribute to her mother -- revived Arquette's big screen career. After tasting success as a movie star in the 1990s, she had shifted her focus to television about 10 years ago.
She called the film a "unique experience" -- the actors filmed for a few days every year for 12 years, following the life of a boy from aged five until going off to college at 18.