Red Sox manager John Farrell announced after Tuesday night's game that knuckleballer Steven Wright was optioned back to Triple-A Pawtucket and added that a corresponding roster move would be made Wednesday.
That move was used to summon Bard, who was one of the top relievers in the American League from 2009-11.
Bard tried to make the transition to a starting role last season but got lost mechanically. That experiment ended in early June, and Bard had trouble regaining his feel and confidence both in Triple-A and when he was recalled by the Red Sox late in the season.
The power righty made some strides during Spring Training, but he was not consistent enough to make the team.
It is unclear if Bard's stint in Boston will be temporary while the Red Sox are short on relievers or if he will stay for an extended period.
Bard pitched a scoreless sixth inning for Portland on Tuesday. He was unscored on in his last five outings and has a 4.00 ERA over eight outings. In nine innings, Bard has four strikeouts and four walks.