Used as a kind of feed additive, p-arsanilic acid can pose a potential risk to organisms when abandoned in the environment. The photodegradation of p-ASA was investigated under UV-C irradiation in the presence of persulfate (PS) in this work. The addition of PS facilitated the decomposition of p-ASA and notably, the presence of 50 mmol PS brought about a nearly complete mineralization after 3 h, while an insignificant total organic carbon (TOC) removal was observed under UV irradiation (λ = 254 nm) only. Experimental results proved that sulfate radical (SO4•-) was responsible for the promotion effect. The cleavage of As-C bond released inorganic arsenic and caused the occurrence of various organic products, for example, hydroxybenzaldehyde, nitrobenzene, benzenediol sulfate, and biphenylarsinic acid. The application of PS with UV-C light may throw a light on thorough treatment for p-ASA containing wastewater.