The deal, made late Saturday, helped the Cardinals kill two birds with one stone.
They obtained a good all-around player who can play either infield or outfield positions and he played for their hated National League Central rivals, the Chicago Cubs, last season.
Many Cubs fans were upset when the news broke.
"We felt that this deal provided us with an excellent opportunity to improve our team, especially from an offensive standpoint," said St. Louis General Manager John Mozeliak.
The acquisition of DeRosa likely ends a dilemma for St. Louis Manager Tony Larussa, who has been looking for a player to fill the void created by the right shoulder surgery on third baseman Troy Glaus, which has kept him off the field this season.
DeRosa has a .270 batting average with 13 homers and 50 RBI in 278 at-bats.