The 27-year-old Middlebrooks spent last season with the San Diego Padres, hitting .212 with nine homers and 29 RBIs.
The Brewers announced the deal Tuesday.
The right-handed hitter was once a promising prospect for the Boston Red Sox, hitting 15 homers and hitting .288 in 75 games in his first year in the majors in 2012.
The rebuilding Brewers are looking to replace Aramis Ramirez at third. Ramirez was dealt to Pittsburgh in July before he retired.