In this paper, we designed and synthesized a novel TBET-based ratiometric fluorescent chemodosimeter, RH-Au, for Au³⁺. It was found that the probe RH-Au displayed highly selective, sensitive and naked-eye detection upon the addition of Au³⁺. The probe RH-Au can be used in the pH range 6.0–7.5 and the detection limit was determined to be as low as 2.91 nM (0.57 ppb). We also demonstrated a successful application of imaging Au³⁺ in living cells using RH-Au.