Water pollution with mercury is a global concern. Therefore, establishing a rapid and accurate detection method is urgently required. Nanosensors can be a perfect alternative to instrument detection. In order to overcome low sustainability of sensors, a new composite nanosensor of sodium alginate- silver nanoparticles (SA-AgNPs) was synthesized by solvent casting method and used in colorimetric and naked eye detection of trace Hg ions in water samples. The structural features of the produced nanosensor were characterized by instrumental techniques. The obtained results confirmed the formation of AgNPs with an average size of 13.34 nm. The colorimetric sensing of Hg was carried out under specific conditions (pH = 6 and reaction time of 7 min) with a linear correlation obtained between the absorbance at 402 nm and different Hg ion concentrations within the range of 0.025 μM-60 μM. The synthesized composite nanosensor of SA-AgNPs detected Hg ions with a detection limit (LOD) of 5.29 nM. In addition, this sensor was successfully applied to detect Hg ions in the environmental water samples with recoveries within the range of 81.58% to 114.73%. The produced nanosensor exhibited good selectivity toward Hg ions in the presence of several competing ions.