With Jacoby Ellsbury sidelined by a partially dislocated right shoulder, the Red Sox were in desperate need of some help in the outfield. Byrd is off to a terrible start this season with just three hits in 43 at-bats, but he hit .276 in 119 games last season and made the NL All-Star team in 2010.
The Cubs also sent cash to Boston in exchange for right-hander Michael Bowden and a player to be named.
The 25-year-old Bowden began this season on Boston's opening-day roster and had a 3.00 ERA in two relief appearances before he was designated for assignment on April 15.
The Red Sox wasted a nine-run lead and lost 15-9 to the Yankees on Saturday.