Prior, thanks to a barrage of injuries, hasn't pitched in the Major Leagues since 2006. The right-hander will start out in extended spring games pitching out of the bullpen.
"We signed Mark Prior to a Minor League deal," said general manager Ben Cherington. "He's been in Fort Myers for a while, working out and throwing in extended games. He looked good, the velocity's been good. He's coming off a groin procedure this winter, so we're going to leave him down there for a while just to build strength before we send him out to a Minor League club. He signed a Minor League deal and will keep pitching in extended [spring games] for a while."
Prior, 31, is 42-29 with a 3.51 ERA in 106 Major League outings, all of which have been starts.