Gallic acid (3,4,5-trihydroxybenzoic acid, GA) is one of the most abundant phenolic acids found in the plant kingdom. In this work, the electron paramagnetic resonance (EPR) spectrum of the gallate semiquinone radical tri-anion (GAS) derived from GA by air oxidation was measured and analyzed by advanced simulation procedures. The observed main spectrum was surrounded by five satellite spectra from which a thorough analysis led to determination of hyperfine splittings (HFS) from five chemically different 13C nuclei in natural abundance. The spectra were further characterized by detailed linewidth analyses. The assignment of the 13C HFS constants was supported by the results of theoretical calculations, using the classical, semi-empirical Karplus-Fraenkel approach, as well as quantum chemical procedures based on density functional theory (DFT), representing the influence of the solvent by polarizable continuum models (PCM). The combined results suggest a consistent assignment of positions and signs for all five 13C constants of GAS, providing a unique insight into the electron spin structure of this radical.