As part of the trade, Colorado will send cash to St. Louis to offset part of the money Arenado is due in his contract.
Arenado had been set to be paid $199 million over the remaining six seasons of a $260 million, eight-year contract. As part of his agreement to waive a no-trade provision, Arenado agreed to add a season to his deal, which now extends for seven seasons through 2027.
His deal had given him the right to opt out and become a free agent after the 2021 season. His new contract gives him the right to opt out and become a free agent after either the 2022 or 2023 season.
"Many have heard me say that one of the great things about baseball is that you always have a chance to get better,'' Cardinals president of baseball operations John Mozeliak said in a statement. "Today we got better! A deal of this nature, acquiring a player of Nolan's considerable talents, are the ones that can set you apart in many ways."
The 29-year-old Arenado has hit .293 with an .890 OPS over eight seasons, averaging 35 home runs and 114 RBIs per 162 games. Aided in part by hitter-friendly Coors Field, he has led the National League in home runs three times and topped the majors in RBIs twice.
The Cardinals finished second in the NL Central last season and lost a first-round playoff matchup against the San Diego Padres. Arenado will bump Matt Carpenter out of his role as the starting third baseman and play in an infield with All-Stars Paul DeJong at shortstop and Paul Goldschmidt at first base.
Arenado slumped during the pandemic-shortened 2020 season, hitting .253 with eight home runs and a .738 OPS over 48 games before going on the injured list Sept. 21 with a bruised left shoulder. He earned $12,962,963 in prorated pay and won his eighth straight Gold Glove. Arenado led the majors with 15 defensive runs saved.
Colorado general manager Jeff Bridich acknowledged last offseason that he was listening to trade offers on the five-time All-Star, and Arenado said in February 2020 that "there's a lot of disrespect around there'' and "there is no relationship anymore'' between him and Bridich.
Like former Rockies stars Troy Tulowitzki and Matt Holliday before him, Arenado grew tired of losing, especially in an NL West division ruled by the Los Angeles Dodgers every year he's been in the big leagues.
St. Louis brings back much of the same team that made last year's postseason, including veteran pitcher Adam Wainwright, who finalized an $8 million, one-year deal Friday. Wainwright is returning for his 17th season with St. Louis, matching Bob Gibson (1959-75) for the second-most seasons with the Cardinals among pitchers, one behind Jesse Haines (1920-37).
Wainwright was 5-3 with a 3.15 ERA last season, striking out 54 in 65 2/3 innings. He is 167-98 with a 3.38 ERA in his career, earning three All-Star selections and two Gold Gloves.
All-Star catcher Yadier Molina remains a free agent. The 38-year-old could still return for an 18th season with the Cardinals after hitting .262 with four home runs in 2020.
In the 10 years since the Platinum Glove Awards were introduced to honor the best fielder regardless of position in each league, Molina and Arenado are tied for the most with four each.
Arenado is the latest established star acquired by St. Louis in his prime.
Mark McGwire came over from Oakland in July 1997 and agreed two months later to a $28.5 million, three-year contract rather than test the open market. Jim Edmonds was obtained from the Angels in March 2000 and reached a $57 million, six-year deal that May. Holliday arrived in a swap with the Athletics in July 2009, became a free agent and agreed in February to a $120 million, seven-year deal.
Most recently, the Cardinals acquired Goldschmidt, a six-time All-Star, from Arizona prior to the 2019 season, and he signed a $130 million deal to stay with St. Louis through 2024.
Gomber was 1-1 with a 1.86 ERA in four starts and 10 relief appearances last year, striking out 27 and walking 15 in 29 innings.
The 20-year-old Gil and the other minor leaguers involved in the deal, all 22, did not play last year because of the pandemic.
Gil was a third-round draft pick by St. Louis in 2018. Locey was taken in the third round and Sommers in the 10th in 2019. Gomber, 27, was a fourth-round selection in 2014. Montero was signed by the Cardinals as an international free agent in August 2014.