Marin County Police said he was pronounced dead at his home shortly after officials responded to an emergency call around noon local time.
Williams was famous for films such as Good Morning Vietnam and Dead Poets Society and won an Oscar for his role in Good Will Hunting.
His publicist said he had been "battling severe depression".
In the past he had talked, and even joked, about his struggles with alcohol and drugs.
Williams had recently returned to a rehabilitation centre to "fine-tune" his sobriety, the Los Angeles Times reported in July.
"At this time, the Sheriff's Office Coroner Division suspects the death to be a suicide due to asphyxia, but a comprehensive investigation must be completed before a final determination is made," police said in a statement.
In a statement, Williams' wife, Susan Schneider, said she was "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 from previous marriages.
Born in Chicago, Illinois in 1951, Williams joined the drama club in high school and was accepted into Juilliard School in New York, the prestigious American academy for the arts.
There, he was encouraged by a teacher to pursue comedy.
The actor was first known for his zany portrayal of an alien in the 1970s TV show Mork and Mindy.
He was a regular stand-up comedian while continuing to act in such films as Good Morning Vietnam, Dead Poets Society, Mrs Doubtfire and as the voice of the genie in Aladdin.
While many of his roles were in comedies, Williams won the Oscar in 1998 for best supporting actor as a therapist in Good Will Hunting.
A senior Japanese stem cell scientist has died in an apparent suicide.
Yoshiki Sasai, who recently co-authored two controversial papers on stem cells, was found dead at his laboratory, the Riken Center for Developmental Biology in Kobe, Japan.
Riken's deputy director, Sasai was renowned for his ability to coax stem cells into becoming other types of cells. This year, however, his career has been under the spotlight. Sasai was a co-author on two research papers that claimed to produce embryonic stem cells called STAP from adult cells using acid. The papers were retracted from the journal Nature in July due to multiple errors.
Sasai, 52, was cleared of any direct involvement by a Riken investigation, but criticised for his failure to correctly edit the papers and for his supervision of lead author, Haruko Obokata, who was found guilty of misconduct in April.
In a letter published on 2 July, Sasai spoke of his deep regret that he was not able to identify the errors in the papers before publication. "Considering the discrepancies that have been pointed out recently... it has become increasingly difficult to call the STAP phenomenon even a promising hypothesis," he said. Experiments to clarify whether or not STAP cells do exist continue at Harvard and Riken.
Nature reports that a bag found at the scene contained three letters addressed to Riken management, Sasai's laboratory members and Obokata.
Philip Campbell, editor-in-chief of Nature said in a statement that Sasai's death is a true tragedy for science and an immense loss to the research community. "Yoshiki Sasai was an exceptional scientist and he has left an extraordinary legacy of pioneering work across many fields within stem cell and developmental biology. Our thoughts are with his family, friends and colleagues at this time."
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