The relationship between the acid–base chemistry and the supramolecular behavior of 5-(4-aminophenyl)-10,15,20-tris(4-sulfonatophenyl)porphyrin in acidic water is reported. A new species exhibiting a prominently red-shifted Q-absorption band at 742 nm is described which is in accordance with a hyperporphyrin-type spectrum of a J-aggregate in water. UV-vis spectroscopy and peak force microscopy reveal that depending on the pH value of the medium the porphyrin self-assembles into two structurally different mesophases which can be reversibly interconverted at will.