The weathering of arsenopyrite is closely related to the generation of acid mine drainage (AMD) and arsenic (As) pollution. Solar radiation can accelerate arsenopyrite oxidation, but little is known about the further effect of SO₄²⁻ on the photochemical process. Here, the photooxidation of arsenopyrite was investigated in the presence of SO₄²⁻ in simulated AMD environments, and the effects of SO₄²⁻ concentration, pH and dissolved oxygen on arsenopyrite oxidation were studied as well. SO₄²⁻ could accelerate the photooxidation of arsenopyrite and As(III) through complexation between nascent schwertmannite and As(III). Fe(II) released from arsenopyrite was oxidized to form schwertmannite in the presence of SO₄^2–, and the photooxidation of arsenopyrite occurred through the ligand-to-metal charge-transfer process in schwertmannite–As(III) complex along with the formation of reactive oxygen species in the presence of O₂. The photooxidation rate of arsenopyrite first rose and then fell with increasing SO₄²⁻ concentration. In the pH range of 2.0–4.0, the photooxidation rate of arsenopyrite progressively increased in the presence of SO₄²⁻. This study reveals how SO₄²⁻ promotes the photooxidation of arsenopyrite and As release in the AMD environment, and improves the understanding of the transformation and migration of As in mining areas.