A strong enhancement of the two-photon absorption (TPA) cross section of tetrakis(4-sulfonatophenyl)porphyrin diacid (H(4)TPPS(2-)) at various wavelengths when a J-type aggregate is formed in water with respect to the one observed for the H(4)TPPS(2-) monomer in a mixture of water, dimethyl sulfoxide (DMSO), and urea is presented. The TPA properties are characterized by the open aperture Z-scan technique and the ultrafast two-photon absorption spectroscopy with white light continuum probe (TPA-WLCP) technique. The observed enhancement is discussed in terms of possible electronic cooperative effects in the aggregate.