Felipe Massa is planning for his retirement after the end of this season.
The 35-year-old Brazilian broke the news at a press conference ahead of the Italian Grand Prix.
Massa began his grand prix career in 2002 with Sauber before he moved to Ferrari in 2006 and latterly Williams.
He missed out on the championship to Lewis Hamilton by one point in 2008 at the final race of the season in his native Brazil.
"I managed to achieve many of my dreams and I am very proud of my career," an emotional Massa said.
Massa, who won 11 times in a career which will yield 250 starts, survived a horrific accident during qualifying at the Hungarian Grand Prix in 2009.
The Brazilian was struck on the crash helmet by a spring which came off Rubens Barrichello's car.
He suffered a fractured skull in the freak incident but miraculously he returned for the season-opening race in Bahrain the following year where he finished second.
Massa moved to Williams in 2014 after eight seasons with Ferrari.
"It is a very sad and emotional day and it is a big decision he has come to," said deputy team principal Claire Williams.
"He is a true gentleman. The spirit he embodies really says everything about the man he is.
"We were so lucky that Felipe agreed to come and join us.
"Thank you for finishing your career at Williams."
Massa, whose father Luis Antonio, wife Rafaela Bassi and son Felipinho, watched the press conference unfold from a packed Williams motorhome, said his plans for next year were unclear.
His departure from Williams does free up a vacancy for 2017 with Jenson Button, out of contract with McLaren at the end of the season, courted by the British team.
At this stage it remains unclear whether the 2009 world champion will leave McLaren, or indeed wish to extend his grand prix career into an 18th campaign.
Massa, who also achieved 16 pole positions and 41 podiums, added: "I must start by especially thanking my wife and my father, my whole family, my manager Nicolas and all the people who have supported me throughout my career.
"Thank you to God for giving me the opportunities I have had in life and, above all, for protecting me.
"A huge thank you as well to everyone I have worked with over the years. Every team I have been a part of has been a special experience, and not only in Formula One.
"I have so many great memories over the years and thank everyone in all the teams I have come through to help me get to where I am today. My career has been more than I ever expected and I am proud of what I have achieved."
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