Birnessite is regarded as an efficient oxidizing agent that would significantly influence the environmental fate of elements such as arsenic. This study compared the chemical and photocatalytic oxidation of As(III) over birnessite. During the chemical oxidation, As(III) was oxidized to As(V), while Mn(IV) was reduced to Mn(II), subsequently forming MnOOH. The coverage of the reactive sites by MnOOH inhibited the chemical oxidation of residual As(III). At pH 5.0, after 360 min of reaction, 61% of As(III) was oxidized to As(V), and the oxidation of As(III) decreased with an increase in pH. The photocatalytic oxidation of As(III), where almost all As(III) could be oxidized to As(V) over a pH range of 5.0–8.0 and 360 min, was much more efficient compared to chemical oxidation. In contrast to chemical oxidation, the formation of MnOOH slightly affected the photocatalytic performance of birnessite. It was demonstrated that •O₂⁻ radicals and holes (h_vb⁺) played an important role in the photocatalytic oxidation of As(III) over birnessite. Our findings confirmed that light dramatically promoted the oxidation of As(III) by birnessite, broadening the understanding of the environmental behaviors of arsenic.