The 37-year-old Japanese reliever will immediately bolster the back end of the Red Sox's bullpen. Statistically, he was considered one of the best relievers in the majors in 2012, posting a 1.75 ERA and a 0.64 WHIP. He struck out 43 and walked only three in 36 innings.
Uehara completed his physical with the Red Sox on Tuesday before the transaction was complete and said he chose Boston because it was the team that had the most interest.
"First, I am really grateful for the Boston Red Sox expressing the strong desire to acquire me and giving me this new opportunity," he said Tuesday through an interpreter. "Now it's my responsibility to meet their expectation and to give my best performance without getting injured."
He missed more than a month with a lat injury last season for the Texas Rangers.
He'll probably be used as a setup man in a Red Sox bullpen that is already looking like a strength. He joins Alfredo Aceves, Daniel Bard, Andrew Bailey, Andrew Miller, Mark Melancon, Craig Breslow, Junichi Tazawa and possibly Franklin Morales.
"I haven't been told any specific role," Uehara said. "My focus is always to give 100 percent to my pitching."
He will wear No. 19, which formerly belonged to Josh Beckett.