Gold nanoparticles have recently gained attention as heterogeneous catalysts in a variety of industrially relevant processes. The catalytic activity of the particles is directly related to the available surface area, which increases with decreasing particle size. However, their stability in solution decreases along with the size, and surface modifications have to be carried out to enable efficient catalysis also for elongated reaction times. To prolong catalyst lifetime and to study the substrate selectivity, we encapsulated colloidal gold nanoparticles in cowpea chlorotic mottle virus cages and catalyzed the reduction of nitroarenes with different substituents. The reduction mechanism has been investigated carefully, revealing the reduction sequence nitro → hydroxylamine → amine to take place. The reduction rate is slowed by the introduction of the diffusion barrier imposed by the virus cage, and a nonconventional relation between electronic effects and reduction rate constants is reported that originates from the limited pore sizes and charged exterior/interior of the virus cage. Finally, a significantly increased stability of the hybrid nanoreactors and their recyclability are demonstrated.