The 30-year-old spent the last five years with the Bantams but turned down a new contract.
He made 183 league appearances for the Valley Parade side, helping them win promotion to League One and reach the League Cup final in 2013.
McArdle told the club website: "I've met the manager for a chat and his objectives are similar to mine."
Find all the latest football transfers on our dedicated page.
Scunthorpe United have signed Bradford City defender Rory McArdle on a three-year deal.