The Dallas Cowboys legends shared the field from 2004 through 2016, before Romo retired and became a CBS NFL analyst.
Witten announced his decision to retire at a news conference on May 3 at The Star in Frisco, Texas. The 11-time Pro Bowl selection and two-time All-Pro played 15 seasons for the Cowboys.
The 36-year old was also the 2012 NFL Walter Payton Man of the Year.
"It's hard to describe how special Jason Witten is," Romo wrote in the letter posted to his Instagram and Twitter accounts. "Sometimes in life you are lucky to come across someone that will change your life. Most of the time you don't realize it at the moment the profound impact and impression someone will have on you. The difference with Jason Witten is that I knew right away the impact he would have on me. Not only was Jason the most talented, humble and hardest working individual on our football team, but he was one of the most genuine, good-hearted people you could ever meet."
"Some things that are etched in my memory are the workouts in the off-season during our younger years. Jason wouldn't give an inch. He had to work the hardest. It was just inside him to give maximum effort every single day. When we were young I remember him running the two hundred yard shuttles, the four hundred yard shuttles, and willing himself to the finish line every day. Jason has in insatiable desire to win and no matter how he felt day in and day out, he was going to finish and win. If I were a betting man then I would bet he never lost one of those races ... not one. The fastest player didn't always win those races but it was all will. Nobody had a stronger will than 82."
"Because of that effort nobody in that facility was allowed to give half effort when your two hundred and sixty pound leader was beating everyone to the finish line. He just couldn't accept someone having an edge on him. He had to succeed . . . he had to win. "He couldn't stand it if a defensive back ever felt like he got the best of 82 that day. He wanted that DB to know that he was in for a long practice or game guarding him. He couldn't accept being average."
"I will also remember him coming to the sidelines trying to be respectful of how tough the quarterback's job was during the games but sometimes he just couldn't control himself. He would say things like, 'Hey 9, I know we ain't got much time back there, but I got HQ fricking Harris guarding me. I'm getting open pretty quickly now, don't be afraid to take a peek if you got the time.' That was code for Jason saying 'I am wide open and I have a bum on me if you haven't noticed.' . . . and sure enough every time I watched the tape, he was always right."
"During big moments in my career, the first guy I was looking for was Jason Witten. I knew where he would be and I knew he would get open and not once, not once did I ever go to him late in the game on a key down when he was one on one and him not deliver. He always came through.'
"In the playoffs in 2014, it was fourth down against the Lions in the fourth quarter . . . needing a touchdown to take the lead and being our last drive of the game we called a timeout. As I stood on the sideline deciding what play we wanted to call in the most pressure packed moment of our season, by brain wanted to call Y option. The problem was the Lions had double-teamed Jason most of the day and especially on key downs and distances. Nothing would be more key than this."
"On the sideline, the coaches and I decided we would call two plays to ensure we got a good call. The problem was as I stood at the line of scrimmage that the Lions changed the defense from what they used all game. Without knowing for sure what they would be in, I changed both plays and went to 'Tennessee, Tennessee.'"
"You see, I didn't know what the perfect play was versus their coverage, but I did know I had the perfect player. Jason made a move that was rare but brought him wide open. Any high school QB could have delivered that ball. The secret to the play was trusting the best player to be the best player . . . and he was."
"Jason, you have set the standard for every player and coach who walks through the Cowboys facility that there's one way to play and there's one way to practice. And guess what, you can be a great teammate and husband and father while doing it."
"To the best I ever played with."
Witten is expected to join ESPN's Monday Night Football team as an analyst. The tight end's former teammates wrote an open letter to the network after his decision was announced.
Sean Lee, Zack Martin, Dak Prescott, Tyrone Crawford, Ezekiel Elliott, Tyron Smith, Travis Frederick, DeMarcus Lawrence, Jeff Heath, Cole Beasley, Dan Bailey and Byron Jones thanked Witten for setting the standard for them, showing them the right way to go about it and making an impact on football and their lives.