He signed a minimum salary benefit deal, according to the Dallas Morning News.
McCown, 36, has played in 62 games, including 10 starts, for five different teams. However, he has not played in a game since 2015. He is 2-8 in his 10 starts.
The Cowboys have three quarterbacks on their roster: Dak Prescott, Kellen Moore and undrafted rookie Cooper Rushon. But they lost Zac Dysert, who has a herniated disc in his back, so they needed to fill that void.
The Cowboys also signed receiver Javontee Herndon, who worked out for the team Friday morning, and they waived running back Jahad Thomas with an injury designation.