He was 63.
"This is a tragic and sudden loss," his PR woman Mara Buxbaum says in a statement.
Robin Williams apparently died due to asphyxia, according to reports but the cause of death has not been confirmed.
Robin Williams, who during university years was roommates with, and a great friend to Christopher Reeve who played Superman, battled a lifelong struggle against depression, alcohol and drugs.
In June Robin Williams checked himself back into rehabilitation for the umpteenth time, and just completed time at the Hazelden Addiction Treatment Center.
South African TV viewers first saw Robin Williams as the space alien Mork during the starting days of South African television in an episode of Happy Days and then later in the series Mork & Mindy. Multiple movie and TV roles followed over the course of the past decades.
Robin Williams' most recent TV role was opposite Sarah Michelle Gellar last year in the comedy The Crazy Ones on M-Net. It was cancelled after the first season.
Robin Williams earned Oscar nominations for Good Morning Vietnam (1987) and Dead Poets Society (1989) and won for 1997's Good Will Hunting.
He filmed and completed roles in four movies that moviegoers will still see him in.
In a statement Robin Williams' wife Susan Schneider says: "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. 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."
Robin William's manager David Steinberg, says in a statement: "Nobody made the world laugh like Robin Williams. My brother, my friend, my soulmate, I will miss you."
20th Century Fox Television (which produced The Crazy Ones) says in a statement: "Robin Williams was a comedy giant, and although we only knew him personally for a season, he was warm, funny and a true professional. His cast and crew both loved him and loved working with him, and our hearts go out to his family and friends. He was one of a kind"
David E. Kelly who produced The Crazy Ones says in a statement: "Robin Williams' talent was legendary. But equally inspiring, perhaps more so, was his kindness and humanity. A gentle soul who touched us all. A very special man, and our hearts are broken".
American's CBS TV network says in a statement: "Our world has lost a comic genius, a gifted actor and a beautiful man. We will remember Robin Williams as one of the unique talents of his time who was loved by many, but also as a kind, caring soul, who treated his colleagues and co-workers with great affection and respect. Our heartfelt thoughts and sympathies go out to his family, loved ones and friends".
Robert A. Iger, chairman and CEO of The Walt Disney Company says in a statement: "We're deeply saddened by the loss of Robin Williams, a wonderfully gifted man who touched our hearts and never failed to make us laugh".
"An incredible actor and a comedic genius, Robin will always be remembered for bringing some of the world's favorite characters to life, from his zany alien in Mork & Mindy to the irascible genie in Disney's Aladdin. He was a true Disney Legend, a beloved member of our family, and he will be sorely missed".
In a statement to TV with Thinus M-Net says "M-Net is saddened by the death of legendary actor Robin Williams. Over the years Robin Williams brought laughter, joy and magic to the millions of viewers who enjoyed his stellar performances on the M-Net family of channels".
"Viewers simply knew that the appearance of his name in a cast list signalled an unforgettable viewing experience. To celebrate the immense contribution of this exceptionally talented star, M-Net Movies has decided to put together a special movie festival in his memory".