Williams was found unconscious and not breathing at his home north of San Francisco. The Oscar-winning actor was battling with depression and, according to initial police reports, may have taken his own life.
"At this time, the Sheriff's Office Coroner Division suspects the death to be a suicide due to asphyxia," police said.
The veteran actor's publicist said the "Good Will Hunting" star he had been battling depression prior to his death.
"Robin Williams passed away this morning. He has been battling severe depression of late," Mara Buxbaum 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."
Heartbroken wife Susan Schneider urged the father-of-three's millions of fans to remember his genius for creating laughter.
"This morning, I lost my husband and my best friend, while the world lost one of its most beloved artists and beautiful human beings," said Schneider, who became the actor's third wife when the couple married in 2011.
Tributes to the actor are pouring in, from colleagues and family alike, including one from US President Obama.
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.
Robin Williams made the world laugh & think. I will remember & honor that. A great man, artist and friend. I will miss him beyond measure.— Kevin Spacey (@KevinSpacey) August 12, 2014
I am stunned and just reeling from the news of Robin Williams' death. He brought so much joy and heart to millions... Just, wow.— Neil Patrick Harris (@ActuallyNPH) August 12, 2014
I can’t believe the news about Robin Williams. He gave so much to so many people. I’m heartbroken.— Ellen DeGeneres (@TheEllenShow) August 11, 2014