Updated — 8:40 p.m. ET
Williams was found Monday "unconscious and not breathing inside his residence," according to a statement from the sheriff's department. Williams' death is being investigated as "suicide due to asphyxia," it said.
"Robin Williams passed away this morning. He has been battling severe depression of late," Mara Buxbaum, press representative, said in a statement. "This is a tragic and sudden loss. The family respectfully asks for their privacy as they grieve during this very difficult time."
Williams' wife, Susan Schneider, asked for "privacy during our time of profound grief."
"This morning, I lost my husband and my best friend, while the world lost one of its most beloved artists and beautiful human beings," Schneider said. "I am utterly heartbroken. On behalf of Robin's family, we are asking for privacy during our time of profound grief. As he is remembered, it is our hope the focus will not be on Robin's death, but on the countless moments of joy and laughter he gave to millions."
Williams had three children: Zachary Pym, age 31; Zelda Rae, age 25; and Cody Alan, age 22.
The actor famously struggled with issues of addiction and mental health for years. He admitted to abusing alcohol and cocaine in the early 1980s, but quit after his friend John Belushi died of an overdose in 1982. He remained sober for more than 20 years but entered rehab in 2006 for alcohol addiction after a relapse.
"It's [addiction] — not caused by anything, it's just there," Williams told ABC in a 2006 interview. "It waits. It lays in wait for the time when you think, 'It's fine now, I'm OK.' Then, the next thing you know, it's not OK. Then you realize, 'Where am I? I didn't realize I was in Cleveland.'"
Williams underwent heart surgery in 2009.
He checked into rehab on July 1, 2014, for "continued sobriety."
Marin County Sheriff said it would hold a news conference Tuesday on its investigation into Williams' death.
Williams won a Best Supporting Actor Oscar in 1998 for his role as a therapist in Good Will Hunting. He was nominated three other times for Best Actor for his roles in Good Morning Vietnam, The Fisher King and Dead Poets Society.
His breakthrough role was as an alien on the ABC television show Mork and Mindy.
President Obama issued the following statement on Williams:
Robin Williams was an airman, a doctor, a genie, a nanny, a president, a professor, a bangarang Peter Pan, and everything in between. But he was one of a kind. He arrived in our lives as an alien — but he ended up touching every element of the human spirit. He made us laugh. He made us cry. He gave his immeasurable talent freely and generously to those who needed it most — from our troops stationed abroad to the marginalized on our own streets. The Obama family offers our condolences to Robin's family, his friends, and everyone who found their voice and their verse thanks to Robin Williams.
The last message Williams posted online was a picture of himself with his 25-year-old daughter, Zelda Rae Williams. He uploaded the picture on July 31 — his daughter's birthday.
Robin Williams checked into rehab on July 1, 2014, for "continued sobriety." An earlier version of this post misstated the date.