The release of mercury into the environment has adverse effects on humans and aquatic species, even at very low concentrations. Pyrene and its derivatives have interesting fluorescence properties that can be utilized for mercury (Hg) ion sensing. Herein, we reported the highly selective pyrene-functionalized silica nanoparticles (Pyr-NH@SiO NPs) for chemosensing mercury (Hg) ions in a seawater sample. The Pyr-NH@SiO NPs were synthesized via a two-step protocol. First, a modified Stöber method was adopted to generate amino-functionalized silica nanoparticles (NH@SiO NPs). Second, 1-pyrenecarboxylic acid was coupled to NH@SiO NPs using a peptide coupling reaction. As-synthesized NH@SiO NPs and Pyr-NH@SiO NPs were thoroughly investigated by H-NMR, FTIR, XRD, FESEM, EDS, TGA, and BET surface area analysis. The fluorescent properties were examined in deionized water under UV-light illumination. Finally, the developed Pyr-NH@SiO NPs were tested as a chemosensor for Hg ions detection in a broad concentration range (0-50 ppm) via photoluminescence (PL) spectroscopy. The chemosensor can selectively detect Hg ions in the presence of ubiquitous ions (Na, K, Ca, Mg, Ba, Ag, and seawater samples). The quenching of fluorescence properties with Hg ions (LOD: 10 ppb) indicates that Pyr-NH@SiO NPs can be effectively utilized as a promising chemosensor for mercury ion detection in seawater environments.