The site-selective C‒H thiocyanation of quinoline has potential application value but remains undeveloped. We report herein an electrochemical C3-thiocyanation of quinoline derivatives under external oxidant-free conditions. The key to success for this reaction is the in situ formation of activated silylquinolinium salts. This method exhibits mild reaction conditions, broad substrate scope, and excellent site-selectivity. The practicality of this protocol is further demonstrated by a scale-up reaction, follow-up transformations, and late-stage thiocyanation of quinoline-based bioactive molecules.