Witten made the announcement in a statement on the Cowboys website.
"The fire inside of me to compete and play this game is just burning too strong," Witten said. "This team has a great group of rising young stars, and I want to help them make a run at a championship. This was completely my decision, and I am very comfortable with it. I'm looking forward to getting back in the dirt."
The Cowboys retained Witten's rights when he was placed on the NFL's reserve/retired list. Witten, 36, missed one game during his career, appearing in 15 games during his 2003 rookie campaign. The 11-time Pro Bowl selection and two-time All-Pro had 63 catches for 560 yards and five touchdowns during the 2017 season.
He sat out during the 2016 season to work on ESPN's Monday Night Football crew. Witten has 1,152 catches for 12,448 yards and 68 touchdowns in 239 career appearances.
"We thank Jason for his many contributions to Monday Night Football and to ESPN over the past year and wish him continued success," ESPN said in a statement. "We have seen many former coaches and players go into broadcasting before eventually returning to the game they love, so we understand Jason's desire to return to the Dallas Cowboys. In the coming weeks, we will determine our MNF plans for the 2019 season."