  Active galactic nuclei (AGN) with jets seen at small viewing angles are the
most luminous and abundant objects in the $\gamma$-ray sky. AGN with jets
misaligned along the line-of-sight appear fainter in the sky, but are more
numerous than the brighter blazars. We calculate the diffuse $\gamma$-ray
emission due to the population of misaligned AGN (MAGN) unresolved by the Large
Area Telescope (LAT) on the {\it Fermi} Gamma-ray Space Telescope ({\it
Fermi}). A correlation between the $\gamma$-ray luminosity and the radio-core
luminosity is established and demonstrated to be physical by statistical tests,
as well as compatible with upper limits based on {\it Fermi}-LAT data for a
large sample of radio-loud MAGN. We constrain the derived $\gamma$-ray
luminosity function by means of the source count distribution of the radio
galaxies (RGs) detected by the {\it Fermi}-LAT. We finally calculate the
diffuse $\gamma$-ray flux due to the whole MAGN population. Our results
demonstrate that the MAGN can contribute from 10% up to nearly the entire
measured Isotropic Gamma-Ray Background (IGRB). We evaluate a theoretical
uncertainty on the flux of almost an order of magnitude.
