For the development of excellent optical probes for mercury(II), a series of simple conjugated polymers that contain phosphorescent iridium(III) complexes as receptors for mercury(II) were designed and synthesized. These conjugated polymers showed energy transfer from the polymer host to iridium(III) complex guest in both solution and the solid state. Unexpectedly, they can work as excellent polymer chemodosimeters for mercury(II) by utilizing the mercury(II)-induced decomposition of iridium(III) complex. They exhibit a pronounced optical signal change with switchable phosphorescence and fluorescence, even when the concentration of a solution of mercury(II) in THF was as low as 0.5 ppb. With the addition of mercury(II), the phosphorescent emission intensity of iridium(III) complexes was quenched completely. As the emission from polymer backbones increased, the emission wavelength was redshifted simultaneously, thereby realizing ratiometric detection. Excellent selectivity toward mercury(II) over other potentially interfering cations was also realized. In addition, an obvious emission color change of polymer solution from red to yellow-green was observed, thus realizing a “naked-eye” detection of mercury(II). More importantly, the solid films of these polymer chemodosimeters also exhibited high sensitivity and rapid response to mercury(II), thereby demonstrating the possibility of the fabrication of sensing devices with fast and convenient detection of mercury(II). The sensing mechanism was also investigated in detail. This is the first report on chemodosimeters based on conjugated polymers with phosphorescent iridium(III) complexes.