Polanco, 32, is expected to replace Pete Alonso at first base and also spend some time as the designated hitter. He has made one appearance at first base in his 12-year career, however, and played nearly 1,000 games at second base and shortstop combined.
The move comes two days after five-time All-Star Alonso agreed to a five-year contract with the Baltimore Orioles.
Polanco became a free agent in November after he declined his player option for 2026.
He batted .265 with 26 homers and 78 RBIs in 138 games last season with the Seattle Mariners. The homers were the second most of his career and the RBI total ranked third.
In the postseason, Polanco had three homers and eight RBIs despite a .208 average in 12 games.
He smacked two homers in a Game 2 victory over the Detroit Tigers in the American League Division Series and then delivered the walk-off, series-ending single in a 3-2 victory in the 15-inning Game 5.
In Game 1 of the AL Championship Series against the Toronto Blue Jays, Polanco had two hits and two RBIs in a Seattle win. The next night, Polanco delivered a three-run homer to help the Mariners take a 2-0 series lead. The Mariners eventually lost the series in seven games.
Polanco has a .263 career average with 154 homers and 570 RBIs in 1,088 regular-season games with the Minnesota Twins (2014-23) and Mariners. He was an All-Star in 2019 for Minnesota.
