An acidic ionic liquid immobilized on a magnetic core has been synthesized through the copolymerization of acidic vinylpyridinium IL with a double-bond-modified magnetic core. The ionic liquid was coated on the magnetic core to form a shell structure that provided easily accessible acidic sites for reactants. The magnetic attraction between the particles enhanced the interactions of the ions on the shell surface, resulting in a high stability. The solid acid exhibited a high activity for biodiesel synthesis from waste oils, with a yield of 99% under mild conditions. The simple magnetic recovery of the prepared catalyst adds to its advantages over other acid catalysts.