The A's transferred infielder Scott Sizemore to the 60-day disabled list Monday to clear a spot on the 40-man roster. Oakland will make a move to put Wells on the 25-man roster once he joins the team in Boston.
Wells spent spring training with Seattle but did not make the team. The Blue Jays claimed him off waivers April 10 and he did not play a game before being designated for assignment.
The 28-year-old Wells hit .228 with 10 homers and 36 RBIs in 93 games with the Mariners last season.
Sizemore is out for the season with a left knee injury.