It's the second time Brent has retired since his conviction for intoxication manslaughter in the crash that killed teammate Jerry Brown in 2012. The Cowboys placed him on the reserved/retired list Friday.
The former Illinois star seemed excited for the upcoming NFL season in a visit to campus in April.
"We're definitely looking forward to making it to the playoffs again and going further," he said, before the Illinois spring game on April 18.
Brent made a statement through his agent Friday, saying "football isn't as much of a priority in my life as other things at this time."
The 27-year-old Brent returned to football briefly last year after serving a 10-game suspension. He played in one game in his first action in almost exactly two years.
Brent retired before training camp in 2013 while he awaiting trial in the case. A 10-year prison term in the conviction in early 2014 was suspended, and he ended up serving about six months in jail.
Brent also had a drunken-driving conviction in Illinois, where he played college football.