Maldonado started behind the plate for Kansas City on Monday against the Chicago White Sox, but he was lifted for a pinch hitter in the sixth inning.
The 32-year-old backstop signed a one-year deal with the Royals in spring training after Salvador Perez was lost for the season. He hit .227/.291/.366 with six homers and 17 RBIs in 74 games for the Royals this year, while also displaying his usual stellar defense behind the plate. Maldonado's seven defensive runs saved through Sunday ranked third in the league.
He'll provide additional catching depth for the Cubs, a team suddenly in need of help at the position.
Cubs catcher Willson Contreras will be placed on the injured list with a strain of the arch muscle in his right foot, team president of baseball operations Theo Epstein told Patrick Mooney of The Athletic. His injury accelerated trade discussions with the Royals, but Chicago hopes Contreras will return after the minimum 10-day stint on the shelf.
Montgomery was moved to the bullpen on a full-time basis this year and struggled to find his groove for the Cubs, posting a 5.67 ERA and 1.78 WHIP with 18 strikeouts and 13 walks over 20 appearances. He's a possible candidate to move into the rotation with the Royals, according to Jeff Passan of ESPN.
The 30-year-old spent parts of the last four seasons with the Cubs, serving as a critical member of their recent playoff teams as both a starter and reliever. He's best known for recording the final out of the Cubs' historic World Series championship in 2016, ending a 108-year drought for the franchise.
The trade is also a homecoming of sorts for Montgomery, who was the Royals' first-round pick in 2008 (36th overall). Montgomery spent his first five professional seasons in the Royals' system, but he never pitched for them in the majors. The Royals traded him to Tampa Bay before the 2013 season as part of the deal that brought James Shields and Wade Davis to Kansas City.