The 30-year-old's current deal with the club was due to expire at the end of the 2014 season, so the Bears were keen to secure the long-term future of the five-time Pro Bowl receiver.
Marshall's deal with the club could be worth up to $30m (£17.8m) but has $23m guaranteed money in the contract, which will keep him at the club until the end of the 2017 season.
The receiver signed the contract live on TV show The View and revealed that he will donate $1m to support mental health awareness, following his previous struggles.
He wrote on his Instagram page: "3 year 30 million dollar extension. 1,000,000 million dollar pledge to The Mental health Community. #BearDown #mentalhealthawareness #trailblazers."
Marshall recorded 100 receptions for 1,295 yards along with 12 touchdowns in the 2013 season.