Optical nonlinearities display many fascinating phenomena, such as frequency mixing, supercontinuum generation, and soliton propagation and enable numerous important applications, such as higher-harmonic generation and optical signal processing. On a different note, many studies of the Purcell effect have shown that complex dielectric environments can strongly enhance or suppress spontaneous emission from a dipole source. In this letter, we demonstrate that the Purcell effect can also be used to tailor the effective nonlinear optical susceptibility. While this is a general physical principle that applies to a wide variety of nonlinearities, we specifically investigate the Kerr nonlinearity, present in most materials. We show theoretically that using the Purcell effect for frequencies close to an atomic resonance can substantially influence the resultant Kerr nonlinearity for light of all (even highly detuned) frequencies. In realistic physical systems, enhancement of the Kerr coefficient by one to two orders of magnitude could be achieved.