Display omitted •Riboflavin-derivative as a photosensitizer was more stable than riboflavin.•Degradation of roxarsone to arsenate can reduce the mobility and environmental risk.•Roxarsone was photodegraded by riboflavin-derivative.•1O2, O2− and excited riboflavin-derivative played a key role in the degradation. Roxarsone (ROX) is a typical organoarsenic feed additive, and most of ROX is excreted unchanged in manure and wastewater. The stability and mobility of ROX result in the environmental risk of arsenic contamination spreading. The degradation of ROX to arsenate (As(V)) can reduce the mobility and environmental risk. Riboflavin is a photosensitizer and can catalyze ROX to As(V), but it is not stable in water environment. In this study, riboflavin-derivative 2′,3′,4′,5′-tetraacetylriboflavin (RTA) was used to replace riboflavin as photosensitizer. The possibility of ROX photodegradation using riboflavin-derivative (RTA) as photosensitizer was investigated. The results showed that RTA was more stable than riboflavin in water environment. ROX was degraded to arsenite (As(III)) by RTA in 90 min under simulated sunlight irradiation and 50 min under natural sunlight irradiation, and further to As(V). The degradation was inhibited under acidic or alkaline pH conditions, while was favorable under neutral pH conditions. The mechanisms of ROX degradation by RTA were further explored. 1O2, O2− and excited RTA played important role in the RTA-sensitized ROX photodegradation. The photocatalytic degradation of roxarsone by RTA can reduce mobility of arsenic and the environmental risk caused by organoarsenicals.