Marshall signed the contract on ABC's "The View" on Monday and tweeted that he will donate $1 million to benefit mental health care.
3 year 30 million dollar extension. 1,000,000 million dollar pledge to The Mental health Community.... http://t.co/t5HFNZKIYl- Machine Marshall (@BMarshall) May 19, 2014
In 2011, Marshall disclosed that he had been diagnosed with borderline personality disorder. The 6-foot-4, 230-pound Marshall has a history of off-field problems, but the Bears have had no complaints.
He had one season left on his current contract for $10 million. He will get $23 million guaranteed with his new deal, a source told ESPN.com's Michael C. Wright.
General manager Phil Emery did not indicate that an extension was imminent during an interview with WSCR-AM 670 Monday morning but had high praise for his star receiver.
"A great player," he said. "Obviously a player we'd like to have here for the long term. He is healthy. He feels great."
Marshall, who was acquired by the Bears in a 2012 trade with the Miami Dolphins, has made the Pro Bowl in both of his seasons with Chicago. He had 118 receptions in 2012 and 100 in 2013. Those totals are the top two receiving seasons in Bears history, and Marshall is the only player in franchise history to register back-to-back 100-catch seasons.
He's also the first player to have consecutive seasons of double-digit touchdown receptions, as he had 12 in 2013 and 11 in '12.
Since being drafted in the fourth round by the Denver Broncos in 2006, Marshall has 712 receptions for 9,050 yards and 57 touchdowns.
The Associated Press contributed to this report.