Cedars-Sinai Medical Center Spokeswoman Sally Stewart said Murphy died at 10:04 a.m. Sunday. She would not provide a cause of death, or any other information.
The Los Angeles Fire Department responded to a call at 8 a.m. Sunday from a home that is listed as belonging to British screenwriter Simon Monjack, who is married to Murphy, spokesman Devon Gale said. Gale said one person was transported to a hospital.
Messages left for Murphy's manager, agent and publicist by The Associated Press weren't immediately returned.
Born Nov. 10, 1977 in Atlanta, Murphy grew up in New Jersey and later moved with her mother to Los Angeles to pursue acting.
Her career started in the early 1990s with small roles in television series, commercials and movies. She is best known for parts in "Girl, Interrupted," "Clueless" and "8 Mile."
Her on-screen roles declined in recent years, but Murphy's voice gave life to numerous animated characters, including Luanne Platter on more than 200 episodes of Fox's "King of the Hill" and Gloria the penguin in "Happy Feet."
She is due to appear in Sylvester Stallone's upcoming film, "The Expendables," set for release next year.
Her role in "8 Mile" led to more recognition, Murphy told The Associated Press in 2003. "That changed a lot," she said. "That was the difference between people knowing my first and last name as opposed to not."
Murphy credited her mother, Sharon, with being a key to her success.
"When I asked my mom to move to California, she sold everything and moved out here for me," Murphy told the AP in 2003. "I was really grateful to have grown up in an environment that was conducive to creating and didn't stifle any of that. She always believed in me."
Kutcher tweeted Sunday morning about Murphy's death: "2day the world lost a little piece of sunshine," Kutcher wrote. "My deepest condolences go out 2 Brittany's family, her husband, & her amazing mother Sharon."