In exchange for the six-year veteran, the Red Sox are sending shortstop prospect Argenis Diaz and pitching prospect Hunter Strickland to Pittsburgh.
The 29-year-old LaRoche was scratched from the Pirates' lineup on Wednesday, with early speculation indicating that he was headed to the San Francisco Giants. The report then changed, saying Boston was LaRoche's destination.
A left-handed hitter, LaRoche owns a .269 career batting average with 123 home runs and 426 RBIs in 775 games. He's topped the 20-homer mark in each of the past four seasons, and currently has 12 this season to go with his .247 average.
Defensively, LaRoche has never played any position besides first base. There, he has a stellar .995 fielding percentage in 732 games.
Diaz, 22, hit .253 for Double-A Portland this season, and is known for his defense.
Strickland went 5-4 with a 3.35 ERA this year for Single-A Greenville.
The Pirates have now traded five starting position players since last July: Jason Bay, Nyjer Morgan, Nate McClouth and Xavier Nady.
