An indolizine–rhodamine-based ratiometric fluorescent probe was designed and successfully synthesized. The probe shows a large Stokes shift (204 nm), high sensitivity and high selectivity. The detection limit was calculated to be as low as 8.76 nM. The probe could quickly (5 min) detect Hg²⁺ over a wide pH range from 5 to 10. Furthermore, it could be used for imaging Hg²⁺ in living cells.