We show that polymer diffusion in polymer nanocomposites (PNCs) is controlled by the architecture of polymer brushes grafted to hard spherical nanoparticles (NPs). At high grafting density, diffusing chains are excluded from the polymer brush leading to greater confinement. However, at lower grafting density, these chains penetrate the brush and diffusion is similar to the hard NP case, compared at the same NP loading. We calculate the effective interparticle spacing (IDeff) by modeling polymer penetration into the grafted brush using self-consistent field theory. When plotted against a confinement parameter (IDeff/2R(g), where R-g is the radius of gyration of the diffusing polymer), reduced diffusion coefficients (D/D-o) fall on a master curve independent of brush architecture. These findings show that brush architecture provides a new route toward controlling polymer dynamics and viscoelasticity of PNCs.