The two sides had been in talks this week and finally got a deal done.
The 31-year-old has played all 162 games in back-to-back seasons with the Mets, also earning his first Silver Slugger Award after hitting 38 home runs with 126 RBI.
Alonso also led the National League with 41 doubles - third most in the entire league, also a career-best.
The five-time All-Star and two-time Home Run Derby Champion posted a personal-best .272 batting average this past season thanks to a career-high 170 hits. He finished 2025 with a .272/.347/.524 slashline - the best of his career.
Alonso has spent all seven years of his career with the Mets after debuting with the team in 2019, a year in which he won Rookie of the Year after hitting a league-best 53 home runs with 120 RBI and a stellar .941 OPS.
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