The news brought an outpouring of mourning - from his family, Hollywood friends and even the White House.
Susan Schneider, Williams' wife, asked in a statement for her husband to be remembered by "the countless moments of joy and laughter he gave to millions."
"This morning, I lost my husband and my best friend, while the world lost one of its most beloved artists and beautiful human beings," she said. "I am utterly heartbroken."
Williams first gained fame as a ridiculous space alien in the hit television comedy "Mork and Mindy" from 1978-82.
Alongside a long career in stand-up comedy, where his manic performances became legendary, he had memorable, often quirky film roles in both comedies and dramas: "Popeye," "The World According to Garp," "Moscow on the Hudson," "Good Morning, Vietnam," "Dead Poets Society," "Awakenings," "Mrs Doubtfire" and "Good Will Hunting," which earned him an Oscar for best supporting actor.
Williams' press representative Mara Buxbaum confirmed that the 63-year-old actor had been battling severe depression and asked for the family's privacy to be respected after the "tragic and sudden loss."
The coroner division of the Marin County Sheriff's Office said it "suspects the death to be a suicide due to asphyxia, but a comprehensive investigation must be completed before a final determination is made."
US President Barack Obama recalled Williams' many roles: "He arrived in our lives as an alien, but he ended up touching every element of the human spirit. 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."
Friends and colleagues took to social media to express condolences.
Steve Martin tweeted: "I could not be more stunned by the loss of Robin Williams, mensch, great talent, acting partner, genuine soul."
Actress Goldie Hawn wrote: "Oh Robin ... Our hearts are broken. Rest in peace darling. We loved you."
The 63-year-old had struggled for decades with addictions to alcohol and drugs. Just last month the star checked into an addiction centre in Minnesota for several weeks, according to multiple media reports
In 2006, he admitted to alcohol abuse and signed up for treatment. He reportedly had given up alcohol in the 1980s before picking up the habit again.
Williams had already shot a reprise of his role as Theodore Roosevelt in the third instalment of Night at the Museum, due for release in December.