The Spaniard has signed a new five-year contract at Stamford Bridge.
The 27-year-old Spain defender joined the Blues in August 2016 and helped them win the Premier League in 2016-17 and claim the FA Cup last season.
Alonso said on Chelsea’s official website: “I am so happy to stay here longer and to keep playing for one of the best teams in the world.
“It’s been a very good two seasons and I am looking forward to more.”
Alonso has made 92 appearances and scored 15 goals for Chelsea since being signed from Fiorentina.
He previously had spells in England with Bolton and Sunderland.
The left-back, who began his career at Real Madrid, made his international debut in March this year and has won three caps to date.
Chelsea director Marina Granovskaia added: “We are delighted to be extending Marcos’ contract.
“In the past two seasons he has developed into an important player for the club, demonstrating his outstanding ability and a fantastic attitude that has helped him become a Premier League champion and Spain international.”
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