Labelling and detecting 5‐formylpyrimidine is essential to understand its biological function. Herein, we describe a visible‐light triggered and catalyst‐free reaction in which 5‐formylcytidine RNA can be selectively labelled with 2‐thioaniline under the irradiation of a commercial blue LED light to generate fluorogenic 5‐(benzothiazol‐2‐yl) cytidine.