The Astros were in the market for a first baseman with Yuli Gurriel struggling this season. The 38-year-old Gurriel hit .319 to win the American League batting title in 2021, but has hit just .243 with seven homers and 28 RBIs this year.
Mancini, who has been in the lineup at designated hitter 51 times this season, also could fill that role for the AL West leaders with Michael Brantley out indefinitely with a shoulder injury.
The 30-year-old Mancini is hitting .268 with 10 homers and 41 RBIs in 92 games this season. He broke into the majors with Baltimore in 2016 and batted .270 with 117 homers and 350 RBIs in 701 games with the Orioles.
Houston also received minor league right-hander Jayden Murray from Tampa Bay. The Astros sent Siri to the Rays and minor league right-hander Chayce McDermott to the Orioles.
The 27-year-old Siri made his big league debut last year. He hit .178 with three homers and 10 RBIs in 48 games with Houston this season.
Mancini has a $7.5 million salary this season as part of a contract that includes a $10 million mutual option for 2023 with a $250,000 buyout.
The AL West leaders also added catcher Christian Vázquez from Boston. Houston sent Boston minor league infielder Enmanuel Valdez and outfielder Wilyer Abreu for Vázquez.