A nanocomposite material characterized by peroxidase-like properties was developed through the dispersion of platinum nanoparticles (PtNPs) inside a hydrogelic matrix. The integration of the PtNP catalysts within the matrix resulted in their stabilization, preventing aggregation and precipitation in media of environmental interest, characterized by high ionic strength and by the presence of organic solutes. A thorough optimization of the matrix design was aimed at granting optimal diffusion of the reagents, in order to maintain the efficiency of the catalytic action. Such combined features allowed the implementation and prototyping of a colorimetric method for the detection of mercury in environmental water samples. The assay was based on a chromogenic reaction catalyzed by the peroxidase-like activity of PtNPs and its specific inhibition caused by trace amounts of mercury.