Germano was designated for assignment when Boston activated center fielder Jacoby Ellsbury from the disabled list last Friday. He threw 5 2/3 scoreless innings against the Yankees in his only appearance with Boston on July 7.
Germano, who turns 30 on August 6, is 8-20 with a 4.91 ERA in 80 major-league games with San Diego, Cincinnati, Cleveland and Boston. He made his major-league debut with the Padres in 2004.
The Cubs designated right-hander Jairo Asencio for assignment to make room for Germano, who is expected to be in uniform for the opener of a three-game series at St. Louis on Friday night.
Chicago announced the deal following its 4-2 victory over Miami on Thursday.