Abstract J-aggregates are supramolecular assemblies of dyes exhibiting strong absorption and fluorescence with narrow linewidths, as well as large optical nonlinearities, induced by the formation of largely delocalized molecular excitons. The degree of cooperativity achievable in J-aggregates ensembles, however, is limited by local disorder and thermally induced decoherence effects. A way to overcome these limitations and increase molecular exciton delocalization and coherence is to couple the ensemble of highly ordered molecular dipoles to a common electromagnetic mode in an optical resonator. In this work, we use dielectric metasurfaces to alter the radiative properties of coupled J-aggregate films and demonstrate a 5-fold Purcell enhancement of the luminesce intensity and narrowing of the emission directivity down to ∼300 mrad around the normal. These results highlight the potential of designer dielectric metasurfaces to foster the emergence of cooperative phenomena in excitonic systems, including optical nonlinearities and superradiance.