Parkin, 30, was under contract until next summer but has agreed terms with the Bluebirds for its termination.
He made 14 League One appearances for the Iron, scoring six goals as he helped them avoid relegation to League Two. He made just 15 appearances in 18 months at Cardiff, netting twice.
Alan Knill, the Iron manager, has said he would like to sign the experienced forward but he is likely to be offered higher wages by other clubs.