The title compound, H3O+·C6H2N3O7 −, consists of one picrate anion and one oxonium cation. The oxonium cation is located on a crystallographic twofold axis and both its H atoms are disordered, each over two symmetry-equivalent positions with occupancy ratios of 0.75. The picrate anions are also located on twofold axes bisecting the phenolate and p-nitro groups. π–π interactions between the rings of the picrates [centroid-to-centroid distances of 3.324 (2) Å] connect the anions to form stacks along the a-axis direction. The stacks are further joined together by the protonated water molecules through hydrogen bonds to form two-dimensional sheets extending parallel to the ab plane. The sheets are stacked on top of each other along the c-axis direction and connected through C—HO interactions between the CH groups of the benzene rings and the picrate nitro groups, with CO distances of 3.450 (2) Å.