The cause of his death is unknown.
"We are heartbroken by the tragic death of Marion Barber III," the Cowboys said in a statement. "Marion was an old-school, hard-nosed football player who ran with the will to win every down.
"He had a passion for the game and love for his coaches and teammates. Our hearts go out to Marion's family and friends during this difficult time."
The Fort Worth Star-Telegram reported that police in Frisco, Texas, responded to a welfare check at an apartment believed to be leased by Barber after someone called about a water leak. Officers then forced their way into the residence.
"Frisco Police responded to a welfare concern at an apartment believed to be leased by Marion Barber," police spokesperson Joshua Lovell said in a statement. "Frisco Police along with the Collin County Medical Examiner's Office are investigating an unattended death at the location."
Barber, a fourth-round pick in the 2005 NFL Draft out of Minnesota, played for the Cowboys from 2005-10 and was named to the Pro Bowl in 2007. During that season, he led the team with 975 rushing yards and 10 touchdowns.
After the Cowboys released him following the 2010 season, Barber spent one year with the Bears. He ended his seven-year career with 4,780 yards and 53 touchdowns on 1,156 carries over 99 games. He also caught 179 passes for 1,330 yards and six scores.
Barber's physical running style earned him the nickname "Marion the Barbarian," and he was a fan favorite among Cowboys supporters.
"The NFL mourns the tragic loss of Marion Barber III and we extend our deepest condolences to his family and loved ones," the league said in a statement.
Since his retirement after the 2011 season, however, Barber's life had been in a tailspin.
In 2014, Barber was detained by police in Mansfield, Texas, for a mental health evaluation. He then was arrested in 2019 on two misdemeanor counts of criminal mischief over a pair of incidents in which he was accused of damaging two cars while running in Frisco.
The two separate incidents occurred in 2018, according to NBC 5 in Dallas-Fort Worth, with a witness alleging that Barber charged toward the vehicles and caused large dents to the side of the cars.
"As I watch this video and me knowing exactly how Marion [Barber's] life is going right now today is why I built @personalcorner," Bryant wrote. "I can't even enjoy it because he's down and out bad. We are just a stat and moments to most people."