A turn on/off rhodamine-BODIPY (RB) fluorescent sensor has been designed for selective and sensitive detection of Hg (II) cation in half-aqueous solution. For this purpose, a BODIPY derivative having dual-cores is bilaterally bound to a rhodamine containing dual-amine moieties. The newly prepared sensor reacts with mercury (II) salt to generate a product with different optical parameters. These are changes confirmed by absorption, emission, and excitation measurements. A blue shift is observed as the 'Turn-off' change the pink color to transparent upon the addition of Hg (II) ions which is assigned to the suppression of fluorescence resonance energy transfer upon mercury (II) ion affinity. Selectivity toward the Hg (II) ion is found to depend upon the cavity size created of the amino moieties of rhodamine and the carbonyls of BODIPY to the metal ion. The sensor RB is selective and sensitive to Hg (II) with a detection limit [1.94 (±0.2) 10-8 M]. The three-way fluorescent probe can be selectively applied for the detection of mercury (II) ion in real water samples.