The deal includes an invitation to big league spring training, team spokeswoman Pam Ganley said Tuesday.
Reyes was 3-1 with a 3.55 ERA in 59 appearances last year with the St. Louis Cardinals, his 10th major league team. The 33-year-old has pitched in at least 50 games in each of the past five seasons and is 35-35 with a 4.18 ERA in 669 career outings.
Boston has been trying to improve its bullpen, which had the AL's third worst ERA last year. The Red Sox have added right-handers Bobby Jenks, Dan Wheeler and Matt Albers and re-signed lefty Hideki Okajima.