The Grubbs–Hoveyda type ruthenium–carbene complexes have been covalently immobilized on both silica coated and uncoated magnetic nanoparticles utilizing an imidazolium salt linker. These MNP-supported catalysts showed comparable catalytic activity to the homogeneous imidazolium salt-tagged ruthenium catalyst in the ring-closing metathesis (RCM) of dienes, and could effectively catalyze RCM reactions in the presence of only 0.85 mol % Ru. The MNP-supported ruthenium catalysts could be easily recovered using an external magnet. The recyclability of the silica-coated MNP-supported catalysts was superior to silica uncoated MNP-supported catalysts allowing reuse 14 times, due to the increased compatibility of the silica-coated MNPs with the reaction medium.