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Degradation of sulfapyridine as a representative sulfonamide by the UV/chlorine process was investigated. Kinetic experiments revealed that aniline and sulfonamide moieties were sulfapyridine’s main reaction sites with •OH, and the aniline moiety was the preferable site for Cl• attack. Effects of four influence factors (pH; humic acid, HA; HCO3 –; Cl–) on the degradation kinetics were explored by response surface methodology, and it was found that pH and HA have the highest level influence (81%) among all influence variables (linear, quadratic, and interactive terms), and low pH and concentrations of HA were beneficial for degradation. Approximately 70% of sulfapyridine degradation was observed within 60 min in the UV/chlorine system (pH = 5) without HA. A kinetic model considering steady-state concentrations of reactive radicals and their second-order rate constants is suitable for predicting sulfapyridine degradation, elucidating that •OH and Cl• are major radicals for sulfapyridine degradation in freshwater, and •OH and Cl2 •– are responsible for the degradation in coastal seawater.