The Spain left-back moved to Stamford Bridge from Fiorentina in 2016 and won the Premier League title in his first season with the Blues.

The 27-year-old, who previously played for Bolton having joined from Real Madrid, also won last season's FA Cup.

"I am so happy to stay here longer and to keep playing for one of the best teams in the world," he said.

"It's been a very good two seasons and I am looking forward to more."

Alonso has scored 15 goals in 92 appearances in all competitions for Chelsea and received his first international call-up in March.

In his debut season, he became the first player in English football league history to win his first 14 starts for a club.

"We are delighted to be extending Marcos' contract," said Chelsea director Marina Granovskaia.

"In the past two seasons he has developed into an important player for the club, demonstrating his outstanding ability and a fantastic attitude."