A straightforward strategy involving photocatalysis has been established for accessing trisulfide dioxides from readily achieved symmetrical tetrasulfides. Stern–Volmer analysis and radical quenching experiments demonstrated the occurrence of a single electron transfer between the photocatalyst and sulfinic acid. Bioactive molecules such as the antihypertensive drug captopril, allicin derivatives, amino acids and terpenes were efficiently and reversibly linked through sulfur–sulfur covalent bonds. Furthermore, flow-setup syntheses of trisulfide dioxides were successfully achieved on the gram scale, indicating the great potential of the developed protocol for practical industrial applications.