A new class of hybrid organic/inorganic molecular catalysts with high, local catalyst concentrations is demonstrated by supporting organic and organometallic catalysts on magnetic nanoparticle based polymer brushes (MPB). Poly(styrene) brushes containing Co(III)-salen or piperazine side chains are prepared via atom-transfer radical polymerization (ATRP) from Fe3O4 nanoparticles modified with appropriate initiator molecules. The polymer brush architecture promotes the cooperative interactions required for Co-salen catalyzed ring-opening of epoxides as demonstrated in the hydrolytic kinetic resolution of rac-epichlorohydrin. In addition, the piperazine functionalized MPB catalyst contains the high catalyst concentration that is required for promoting the Knoevenagel condensation of benzaldehyde and malononitrile with this type of amine catalyst. All the MPB catalysts were easily removed from solution via application of a magnetic field, allowing straightforward recovery and reuse. The versatile MPB architecture can be used to create a variety of recoverable supported organic or organometallic catalysts.