The Cowboys put Brent on the reserved/retired list on Friday, according to the Associated Press.
Brent was convicted of intoxication manslaughter in the death of 25-year-old teammate
As he prepared for his impending trial, Brent decided to retire for the first time before the start 2013 season. He attempted to make a comeback last season and was suspended for 10 games as part of his conditional reinstatement to the NFL.
"It is with careful consideration, much thought and reflection that today I announce my retirement from the NFL," Brent said in a statement via ProFootballTalk. "I have made the decision that Football isn't as much of a priority in my life as other things at this time."
Brent, 27, appeared in one regular season and two playoff games for the Cowboys last year.
"He's going to continue to get his life back in order," Cowboys Coach Jason Garrett said Friday at a new conference. "And I admire Josh in a lot of ways for coming back from a real significant situation in his life. It's not easy and I think he's worked very hard to find some normalcy in his life."
The Cowboys re-signed Brent to a one-year extension in November that would have put under contract through the 2015 season.
As a pro, Brent appeared in 40 games for the Cowboys, making five starts, and had 46 tackles with 1.5 sacks.
Brent was drafted by the Cowboys with a seventh-round pick in the 2010 supplemental draft out of Illinois. In three years with the Fighting Illini, Brent had 71 tackles -- 17.5 for a loss -- with five sacks in 33 games.