The 34-year-old Nixon has a .274 career batting average with 137 home runs and 555 RBIs in 12 seasons with Boston, Cleveland and the New York Mets.
Nixon was a member of the 2004 Red Sox team that won the World Series.
General manager Doug Melvin says Nixon is a veteran who will bring energy and experience to the Brewers and will provide another much needed left-handed bat to the lineup.
Milwaukee also agreed to a minor league contract with outfielder Chris Duffy.
Duffy spent parts of three seasons with the Pirates from 2005 to 2007, but was limited following shoulder surgery last year.