Midfielder goes out on a season long loan.
"We're really pleased to welcome Ryan to the club," said Steve Gallen, The Lions' Director of Football. "His qualities and leadership both on and off the pitch only strengthens our midfield.
"He likes to get on the ball and make things happen, and having made over 100 appearances for Cardiff City - as well as at Crewe Alexandra - Ryan arrives with great experience and knows what it takes to compete at this level.
"Ryan's character is exactly what we look for in a player and I look forward to seeing him join the squad. I wish him the very best of luck for the season ahead."