The Cardinals acquired shortstop Julio Lugo and cash considerations from the Red Sox for outfielder Chris Duncan and a player to be named later or cash considerations.
The move came just hours after Boston snagged first baseman Adam LaRoche from the Pirates in a deal for prospects.
Lugo, 33, was designated for assignment on July 17 and hit .284 with a home run and eight RBIs in 37 games for the Red Sox this season.
"We felt that Julio could help our club by improving our middle-infield depth," Cardinals vice president/general manager John Mozeliak said in a news release. "He gives us another right-handed bat and provides [manager] Tony [La Russa] with additional flexibility. We're optimistic that he will respond positively to a fresh start here in St. Louis."
Duncan, who will be optioned to Triple-A Pawtucket, batted .227 with five home runs and 32 RBIs in 87 games for the Cardinals this season, ranking second on the club with 41 walks. The 28-year-old was optioned to Triple-A after Tuesday night's game and made 68 appearances (64 starts) in left field as well as six at first base for St. Louis.
Red Sox Send Lugo To Cards For Duncan