The quarterback agreed a deal worth up to $108m (£71m) over six years, keeping him with the Cowboys until the end of the 2019 season.
The 32-year-old said in a statement: "This is an organisation that always gives us a chance to win, and that is always the main focus here. We haven't had the type of success that we have all wanted the last few years, but I do know that we are on the verge of doing that.
"This football team that we have is a good team and with all the people we have coming back and the things we are doing behind the scenes, it will make us a very difficult ball club to beat.
"I am excited that ownership and the organisation believes in me to get this job done. Our goal is the Super Bowl, and I am determined and honored to be the guy in this position to help our team do that."
Romo signed for the Cowboys in 2003 and took over as starter in 2006.