Cowboys head Coach Jason Garrett said Ryan was dismissed after the Dallas defense misfired down the stretch of the 2012 campaign as the Cowboys finished their last six games 3-3 and missed the playoffs.
"I want to express my appreciation to Rob for all of his efforts and contributions to the Cowboys over the past two years," Garrett said in a statement. "At this time, the decision has been made to move forward in a different direction philosophically on defense.
"I have an immense amount of respect for Rob as a person and as a football coach and I wish him and his family the very best."
Ryan's squad allowed 25 points per game this past season, ninth-most in the league, and finished 19th in total defense, Fox Sports said.
Ryan has also served a defensive coordinator for Cleveland and Oakland and was linebackers coach for New England during his 12-year NFL career.