Acidic azo dyes are widely used for their vibrant colors. However, if their photophysics were better understood and controllable, they could be integrated into many more applications such as photosensing, photomedicine, and nonlinear optics. Here, the proton-controlled photophysics of a widely used acid, hydrazo dye, acid violet 3 (AV3) is explored. Density functional theory is used to predict the ground- and excited-state potential energy surfaces, and the proposed photoisomerization mechanism is confirmed with spectroscopic experiments. The ground-state and first two excited-state surfaces of the three readily accessible protonation states, AV3-H, AV3, and AV3+H, are investigated along both the dihedral rotation and inversion coordinates. The deprotonated AV3-H undergoes photoisomerization with blue light (λex = 453 nm) through a dihedral rotation mechanism. Upon the formation of the cis-isomer, the reversion of AV3-H is predicted to occur through a mixed rotational and inversion mechanism. In contrast, AV3 and its protonated form, AV3+H, do not undergo photoisomerization because there is no driving force for either the rotation or inversion of the azo bond in the excited state. In addition, when the azo bond is acidic, the ground-state dihedral rotation reversion mechanism barrier is lower. The mechanistic insights gained here through the combination of theory and experiment provide a roadmap to control the reactivity of AV3 across 11 orders of magnitude of proton concentration, making them interesting candidates for a range of pharmaceuticals.