Noncovalent interactions play a constitutive role in numerous biological processes, including biomolecular adhesion, recognition, and transport. Unlike pi-pi and cation-it interactions, anion-pi interaction was only recently and computationally recognized to participate in biomolecular science. Here, we present the first experimental identification and direct quantification of the nanomechanics of anion-pi interaction in aqueous solution by using a surface forces apparatus with complementary computational simulations. Anionic phosphate ester and pi-conjugated catecholic moieties that abound in marine bioadhesives were used as the model system. Robust adhesion was observed and attributed to anion-pi interaction, which was found to be enabled by the cooperative cation-pi interaction due to the coexistence of a cation. The anion-pi interaction strength follows the trend phosphate ester > HPO42- > SO42- > NO3, affected by charge density, polarity, and hydration effect.