The deal is $6.7 million, sources familiar with the deal told ESPN.
"Carlos is a pro who is always ready to play and will add consistent at-bats from both sides of the plate and quality defense," Pirates general manager Ben Cherington said in a statement. "We look forward to welcoming Carlos to our team and clubhouse."
Santana, 36, finished with a league-average OPS last year, hitting .202/.316/.376 between stints with the Seattle Mariners and Kansas City Royals. But his expected numbers, based on how hard he hit the ball (an 81st-percentile exit velocity) and his elite walk rate (97th percentile), projected far better results, something that drove his market.
Further, nobody was shifted a higher percentage last year than Santana, who saw altered defense in 356 of his 362 left-handed batting appearances. With the ban of the shift coming in 2023, the switch-hitting Santana could see a significant benefit.
After a midseason trade to the Mariners, Santana emerged quickly as a leader, something the Pirates -- whose oldest position player on the 40-man roster is 31-year-old Ji-Man Choi, for whom they traded earlier this winter -- desperately need.
Santana is entering his 14th season and has a career line of .242/.359/.432 with 278 home runs and 925 RBIs. He will rejoin Pirates manager Derek Shelton, who was the big league hitting coach for Cleveland when Santana joined the organization following a trade from the Los Angeles Dodgers in 2008.
Still a strong defensive first baseman, Santana is best known for his plate discipline and power. He has posted walk rates of greater than 13% in every big league season and hit at least 18 home runs in each of his 11 full years in the major leagues.
The Pirates, whose payroll was among the bottom five in baseball this year for the fifth consecutive season, are expected to further to add via free agency or trades this winter, a young core led by center fielder Bryan Reynolds, shortstop Oneil Cruz, and third baseman Ke'Bryan Hayes. Starter Mitch Keller and closer David Bednar anticipate the arrival of catcher Henry Davis, second baseman Nick Gonzales and right-hander Quinn Priester.