He was 63.
Police said that Williams was found unconscious around noon in his home in Tiburon, California, near San Francisco.
Williams was best known for his starring roles in classic comedies like "Mrs. Doubtfire," "Good Morning, Vietnam" and "Jumanji," but also in acclaimed dramas such as "Dead Poets Society." He won the Academy Award for Best Supporting Actor for his portrayal of Dr. Sean Maguire in "Good Will Hunting." He rose to fame while playing Mork the alien in the TV show "Mork & Mindy," a "Happy Days" spinoff.
In "Dead Poets Society," Williams plays John Keating, an electric English teacher at an elite all-boys high school. In a quintessential speech, Keating tells his students:
To quote from Whitman, "O me! O life!... of the questions of these recurring; of the endless trains of the faithless... of cities filled with the foolish; what good amid these, O me, O life?" Answer. That you are here - that life exists, and identity; that the powerful play goes on and you may contribute a verse. That the powerful play *goes on* and you may contribute a verse. What will your verse be?
Williams in Dead Poets Society.
Susan Schneider, the actor's wife, released the following statement to the New York Times' Dave Itzkoff:
"This morning, I lost my husband and my best friend, while the world lost one of its most beloved artists and beautiful human beings. 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 that the focus will not be on Robin's death but on the countless moments of joy and laughter he gave to millions."
Williams guest-hosted "Saturday Night Live" in 1984. He's shown here with SNL stars Eddie Murphy and Joe Piscopo, via AP.
In 2006, after 20 years sober, he checked himself into rehab for alcoholism. He opened up about his struggles with addiction to alcohol and cocaine in a powerful interview with The Guardian and on "Good Morning America."
"It's not caused by anything, it's just there," he said. "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.'"
Last month, he spent time at Hazelden Addiction Treatment Center in a continued sobriety program. His publicist told HuffPost at the time that he was doing well.
Here is the full press release on his death, courtesy of Marin PD.
Fellow actors took to Twitter to express deep sorrows on the death of the popular actor.
"I could not be more stunned by the loss of Robin Williams, mensch, great talent, acting partner, genuine soul," said Steve Martin.
"Terrible, terrible news," Fred Willard tweeted. "Comedy has lost a great man."
"Shocked by the news of Robin Williams passing. Rest in peace my friend," said Albert Brooks.
Williams hosting the Academy Awards in 1996, via AP.
Though Williams was most celebrated for his acting career, he is also remembered his charitable endeavors. Williams spearheaded Comic Relief, which holds concerts and variety shows to raise money to help the homeless.
In a statement, President Obama said that Williams "was one of a kind... 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 actor was also an avid gamer and named his daughter Zelda after the popular video game character. In his last post to Instagram, he paid tribute to his young daughter on her birthday. Zelda Williams is also an actress.
Williams was born in Chicago, Illinois and studied acting at the Juilliard School in New York City.
Williams in "Mrs. Doubtfire."
Need help? In the U.S., call 1-800-273-8255 for the National Suicide Prevention Lifeline.
Here are some of our favorite Robin Williams memories: