"It was a very difficult decision. I went back and forth a number of times," Romo said on a conference call.
Romo said the Houston Texans were at the top of his wish list if he kept playing, but the CBS offer was too good to pass up.
"It really had nothing to do with the Texans and everything to do with CBS," Romo said. "I felt like it was the right decision. My wife would tell you we've had a lot of late nights. It was nice to have some clarity."
Romo hasn't come out and said he's retiring.
"Do I envision playing football? Absolutely not," he said. "Do I expect to get some calls? Yes, that's the reality."
One NFL executive told ESPN via text message that "Romo is now every team's emergency backup QB in case your starter gets hurt" and that those teams would have to "pay him to come out of 'retirement.'"
Romo said: "You never say never," but added that "there's no part of me that wants to play."
With CBS, Romo will become the No. 1 color commentator -- replacing former NFL quarterback Phil Simms -- alongside play-by-play veteran Jim Nantz. He also had drawn interest from Fox and NBC.
"Going from one legendary team to another as I begin the next phase of my career is a dream come true," Romo said in a statement. "I have always known that once my playing career was over I wanted to become a broadcaster. I am ecstatic for the opportunity to work with Jim as I learn the craft and convey to fans my passion for this great game."
Romo said on the conference call that he expects his new broadcasting job to be difficult, but "I've got to attack this just like football."
As the network's No. 1 color commentator for the NFL, Romo will work with Nantz on Sunday afternoon and Thursday evening games. He also will be in line to work CBS's coverage of Super Bowl LIII in Atlanta in February 2019.
Simms has been in the booth since 1995, first for NBC and then CBS after it won the rights to broadcast the AFC. It is not known whether Simms will take another football role at CBS or be out altogether. He also works on Showtime’s “Inside the NFL,” but that position may be connected to having one at CBS.
CBS Sports Chairman Sean McManus said the network is in ongoing discussion with Simms about his next role, but declined to discuss specifics.