Undrafted out of SMU, McCann, 24, made his NFL debut last November, returning an interception 101 yards for a touchdown in the Cowboys' 33-20 victory over the New York Giants. The next week, he ran back a punt 97 yards for a touchdown in a win over the Detroit Lions.
He played nine games for Dallas last season and two this season before the Cowboys released him on Sept. 20.
For the Ravens, cornerbacks Domonique Foxworth is on season-ending injured reserve with a knee injury, and Jimmy Smith (ankle) and Chris Carr (hamstring) have missed time, although both hope to return after the bye week.
To create a roster spot for McCann, the Ravens cut linebacker Prescott Burgess.