The research on ferroelectric materials-mostly inorganic compounds or organic polymers-is increasingly motivated by both basic scientific concerns and the potential for practical applications in electronics and optics. Ferroelectricity in organic solids would be important for the development of all-organic electronic and photonic devices. The conventional approach to making organic ferroelectrics is based on the use of polar molecules. Here we report that through supramolecular assembly of nonpolar conjugated molecules, a remarkable ferroelectric response can be obtained in co-crystals of low-molecular-weight organic compounds. Co-crystals of phenazine and chloranilic acid reveal large spontaneous polarization and sizable room-temperature dielectric constants exceeding 100. The present findings provide an approach to making potentially useful organic ferroelectric materials.