The transient absorption (TA) and two-photon (TPA) absorption spectra of a water-soluble porphyrin, 5,- 10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H4TPPS2-) diacid, in the monomeric and J-aggregated form are reported. TA spectra, measured with ultrafast laser pulses, were excited at 403 nm and probed at wavelengths varying in the 440-1100 nm range. While the TA spectrum of the J-aggregate form has been already investigated, but only in the 450-750 nm region (Kano, H.; Kobayashi, T. J. Chem. Phys. 2002, 116, 184), the TA spectrum of the monomer has never been investigated previously. The TPA spectrum, also measured with ultrafast laser pulses in the wavelength range of 380-440 nm, shows an increase of the TPA crosssection per single molecule by a factor of 30 upon aggregation. Even more intriguing is the appearance of a two-photon allowed transition around 410 nm in the TPA and TA spectra of the J-aggregate form, not present either in the TPA or in the TA spectra of the monomer. The physical nature of the two-photon allowed state giving rise to such a band is thoroughly discussed, in relation to the peculiar excitonic properties of molecular aggregates, and it is tentatively attributed to a bi-exciton state.