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A unique rhodamine-based derivative, 2-amino-3-(((1E,2E)-2-((3′,6′-bis­(diethylamino)-3-oxospiro­[isoindoline-1,9′-xanthen]-2-yl)­imino)­ethylidene)­amino)­maleonitrile (RhGDM), was easily fabricated just by Schiff-base condensation. RhGDM displays superb selectivity and sensitivity for specifically detecting the biologically and ecologically important Ga3+ and Hg2+ cations via a fluorescence “off–on” type in aqueous medium with high quantum yields and also the ultralow detection limits down to nanomolar level, which are far below the maximum permissible dosage values set by the WHO, indicating high efficiency and reliability of the chemoprobe. Meanwhile, RhGDM exhibits Hg2+-selective chromogenic behavior, and Hg2+ can be detected by naked eye. Interestingly, the plausible sensing mechanism is the aforementioned metal cation-promoted hydrolysis of the rhodamine-based Schiff-base skeleton, which was confirmed by the mass spectra, IR spectra, and density functional theory calculations as well as the irreversibilities of fluorescence signals in the corresponding reaction systems, and it is the first reported smart chemoprobe capable of recognizing simultaneously Ga3+ and Hg2+ through the hydrolytic mode to the best of our knowledge. Notably, the analytical applicability of RhGDM can be further realized by using paper films and silica-coated slides. More excitingly, RhGDM was successfully employed in tracing out the intracellular Ga3+ and Hg2+ cations in living cells.