  We report our search for quasi-periodic signals in long-term optical and
$\gamma$-ray data for the blazar PKS~1222+216, where the data are from the
Steward Observatory blazar monitoring program and the all-sky survey with the
Large Area Telescope onboard the {\it Fermi Gamma-ray Space Telescope},
respectively. A quasi-periodic signal, with a period of $\simeq$420\,days and a
significance of $>5\sigma$, is found in the measurements of the optical linear
polarization degree for the source, while no similar signals are found in the
optical and $\gamma$-ray light curves covering approximately the same time
period of $\sim$10\,yr. We study the quasi-periodic variations by applying a
helical jet model and find that the model can provide a good explanation. This
work shows that polarimetry can be a powerful tool for revealing the physical
properties, in particular the configuration of the magnetic fields of jets from
galactic supermassive black holes.
