Background: Mercury (Hg) is a highly neurotoxic pollutant present in different environmental matrices. Herein, a simple and sensitive assay is proposed for Hg detection in environmental water samples employing polyethylene glycol monododecyl ether (PGME) stabilized silver nanoparticles (PGME-AgNPs). Methods: The prepared PGME-AgNPs were characterized by absorption, scanning electron microscopy (SEM), atomic force microscopy (AFM), dynamic light scattering (DLS), and Zeta potential measurements. The addition of Hg(II) to an aqueous matrix changed the color of the sensor, following a decrease in surface plasmon resonance (SPR) band intensity. Results: The detected response was proportional to Hg (II) concentrations and the analytical response comprised a change in absorbance versus concentrations from 4.0 to 24 × 10-8 mol L-1 (0.8 – 4.8 μg L-1) and the limit of detection (LOD) was 4.0 nmol L-1 (0.08 μg L-1). Additionally, the sensor was integrated with the RGB color values of a smartphone, enabling its use as a portable sensor for rapid Hg(II) at a concentration level ranging from 6.0 to 24 × 10-8 mol L-1 (1.2 - 4.8 μg L-1). Spectrophotometric and RGB color value-based approaches were applied for the quantification of Hg(II) in real water samples with satisfactory recoveries ranging from 98.5 to 105%. Conclusion: The proposed colorimetric method with a smart assisted approach was proven a very simple, and quick method, demonstrating practical applicability for on-site Hg screening of aqueous matrices.