A novel‐donor‐π‐acceptor‐type conjugated polymer based on the benzo[c][1,2,5]selenadiazole moiety and the fluorene group via an alkyne module is prepared by the the Sonogashira–Hagihara reaction. The polymer displays an absorption peak at a longer wavelength at 477 nm and shows orange fluorescence at 576 nm. The responsive optical properties of the polymer on various metal ions are investigated by fluorescence emission and UV–vis absorption spectroscopy measurements. Compared with other metal cations, Hg2+ shows the most pronounced UV–vis absorption and fluorescence response of the polymer. The visible color and the fluorescence color of the polymer sensor both disappear after addition of Hg2+, which can be easily detected by the naked eye. The results indicate this kind of polymer containing benzoselenadiazole units can be used as a highly selective and sensitive chemosensor for Hg2+ detection.