The 27-year-old Brazilian was voted the Blues’ player of the year last season, and his new deal runs until 2020.
“I’m very happy to sign a new contract with Chelsea. It’s a dream to continue with this club and over the next four years I will try to help the team win more trophies,” Willian said on the club’s official website.
Chelsea bought Willian from Russian side Anzhi Makhachkala three years ago in a £32million deal, clinching his signature ahead of London rivals Tottenham.
The midfielder went on to become an integral part of the Blues team, helping them win both the Capital One Cup and then the Barclays Premier League title in 2015, scoring 19 goals in 140 appearances.
Willian, though, is determined to continue his impressive displays at Chelsea under new first-team head coach Antonio Conte.
“It’s been an amazing time for me since I joined the club three years ago, I have won titles and last season I was named Chelsea Player of the Year,” he added.
“I hope to be able to stay at that level during the coming seasons and achieve more success.”