In summary, we reported a new rhodamine-bsed turn-on colorimetric and fluorescent Hg2+ sensor (RATU) with excellent overall performance, such as nearly pure water working medium, high sensitivity and selectivity, wide linear Hg2+ concentration range, low detection limits, strong interference immunity, favorable working pH span, naked-eye observed color changes, low cytotoxicity and good validity for signaling Hg2+ in real water sample and living cell. The fact that the RATU-containing cellulose disc could monitor Hg2+ in 100% aqueous solution by the naked eye revealed RATU's promising prospect in the portable onsite detection of Hg2+. Job's plot and reversibility experiments as well as LC–MS, IR and 1H NMR analysis results infer a new interaction way between Hg2+and RATU which is beneficial to novel rhodamine-bsed chemosensor design.