A color changeable cellulose triacetate (CTA) membrane-based optode has been developed for the highly selective and sensitive determination of Hg(II) ions in aqueous solutions. The sensing layer of the optode consists of phenylthiosemicarbazide derivative of rhodamine 6G immobilized in a plasticized CTA membrane. The colorless optode film on reaction with Hg(II) turned pink, in color, and the intensity of the color was found to be directly proportional to the amount of Hg(II). The sensing mechanism is based on the mercury-induced spirolactum ring opening followed by its cyclization to form highly conjugated system which leads to the development of pink color. Various experimental parameters such as pH, optode composition and response time have been optimized. Under these conditions, the sensor showed a wide linear dynamic range of 10–5000ng mL⁻¹ Hg(II), with a detection limit of 1.3 ng mL^–1 . In addition to its stability and reproducibility, optode showed a high selectivity toward Hg(II) ions as compared to other coexisting ions in real water samples. The developed optode is suitable for rapid and onsite detection of Hg(II) ions in a variety of aqueous samples. The sensor was successfully applied for the determination of Hg(II) ions in groundwater samples.