A new rhodamine-based dual signaling probe (L3) has been found to display quick responses through visible colorimetric changes as well as fluorogenic properties on selective 1:1 binding to Hg2+, as delineated by absorption and fluorescence titrations as well as by Job′s method and ESI-MS+ studies. The fluorescent probe L3 displays a 252-fold fluorescence enhancement on binding to Hg2+. A Benesi–Hilderband fit of the absorption titration data gives Kd = (32.01 ± 0.74) μM and a 1:1 binding stoichiometry. Owing to its solubility in mixed organic/aqueous solvents as well as its cell permeability, L3 could be used for in vitro/in vivo cell imaging of Hg2+ with no or negligible cytotoxicity. The detection limit of Hg2+ was calculated by the 3σ method to be 9.28 ng L–1. Moreover, the fluorescence of the L3–Hg2+ system was turned off and its red colour disappeared when KI was added. Thus, the “OFF–ON–OFF” fluorescence sensing behaviour of L3 observed in the presence of Hg2+ and I– may find applications in devices with logic gate functions.