Bako played for the Cubs in 2003-4, helping them win the NL Central and reach the NL Championship Series during his first season in Chicago.
The 36-year-old spent last year with Cincinnati -- his 10th team in 11 seasons -- and batted .217 while setting career highs with 30 runs scored, 35 RBIs and six home runs in 99 games. In Chicago, he replaces the departed Henry Blanco as Geovany Soto's backup.
The left-handed hitting Bako has a .231 career batting average with 94 doubles, 21 homers and 186 RBIs in 745 games.
He could earn $300,000 in performance bonuses based on games started.