Knowledge of atomistic structures at solid/liquid interfaces is essential to elucidate interfacial processes in chemistry, physics, and materials sciences. The (root 3 x root 7) structure associated with a pair of sharp reversible current spikes in the cyclic voltammogram on a Au(111) electrode in sulfuric acid solution represents one of the most classical ordered structures at electrode/electrolyte interfaces. Although more than 10 adsorption configurations have been proposed in the past four decades, the atomistic structure remains ambiguous and is consequently an open problem in electrochemistry and surface science. Herein, by combining high-resolution electrochemical scanning tuning microscopy, electrochemical infrared and Raman spectroscopies, and, in particular, the newly developed quantitative computational method for electrochemical infrared and Raman spectra, we unambiguously reveal that the adstructure is Au(111)(root 3 x root 7)-(SO4 center dot center dot center dot w(2)) with a sulfate anion (SO4*) and two structured water molecules (w(2)*) in a unit cell, and the crisscrossed [w center dot center dot center dot SO4 center dot center dot center dot w](n) and [w...w...](n) hydrogen-bonding network comprises the symmetric adstructure. We further elucidate that the electrostatic potential energy dictates the proton affinity of sulfate anions, leading to the potential-tuned structural transformations. Our work enlightens the structural details of the inner Helmholtz plane and thus advances our fundamental understanding of the processes at electrochemical interfaces.