The 58-year-old Martz replaces Ron Turner, who was fired in January.
Martz was offensive coordinator for St. Louis in 1999, the year the Rams won the Super Bowl. He became the team's head coach the following year and had a 56-36 record in 5 1/2 seasons.
While coaching the Rams, Martz hired current Bears Coach Lovie Smith as defensive coordinator.
After leaving the Rams, Martz became offensive coordinator in Detroit (2006-08) and San Francisco (2008).