It's the second straight season that Houston has acquired Maldonado, 32, at the trade deadline, picking him up from the Angels last year. Maldonado became a free agent after the season.
The Cubs just traded left-hander Mike Montgomery to the Royals for Maldonado because starting catcher Willson Contreras was on the injured list. He returned, and with Victor Caratini entrenched as the backup, Maldonado became expendable.
Maldonado has been one of the best defensive catchers in baseball this season. After signing a one-year, $2.5 million deal with Kansas City, he started in place of Salvador Perez, who is out for the year while recovering from Tommy John surgery.
The Cubs might not be done dealing. They could acquire another catcher, as insurance in case of injury, but only one whom they could stash at Triple-A Iowa until rosters expand in September. They couldn't do that with Maldonado.
Maldonado appeared in only four games for the Cubs and didn't have a hit. Houston's backup catcher Max Stassi is hitting .167.
Kemp, 27, batted .227 with seven homers and 17 RBIs in 186 plate appearances for the Astros.
ESPN's Jesse Rogers contributed to this report.