The 26-year-old defender joined the Blues on deadline day last summer, before going on to become an integral member of Antonio Conte's much-lauded switch to a 3-4-3 formation, which saw Chelsea go on a memorable 13-game winning streak on their way to claiming the Premier League crown.
"It was a very good first year but it was not easy at all!," Alonso told the club's official website.
"There was a lot of work and a lot of training behind that. All the team did great last season and we have to do the same now - work as we did last year, train hard and for sure with the players we have the results will come again this season.
"We want to have a better year than we did last year."
Alonso made 31 appearances for Chelsea last term, scoring a valuable six goals from left wing-back.