The 27-year-old, voted the London club's Player of the Year last season by fans and team mates, said it was "a dream to continue".
"Over the next four years I will try to help the team win more trophies," he added.
Willian joined Chelsea in 2013 and was the standout performer last season in what turned out to be a wretched campaign for the defending champions. He has scored 19 goals in 140 appearances.
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Chelsea, who finished 10th in the league and out of European competition, will have a new manager next season with the arrival of Italian Antonio Conte.
(Reporting by Alan Baldwin, editing by Toby Davis)
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