Highly selective adsorbent for Hg2+ ions was synthesized via 3-amino-1,2,4 triazole,5-thiol (ATT) functionalization of p-phenylenediamine (p-PDA) based covalent organic polymer. The obtained adsorbent was abbreviated as ATT@PDA-COP and successfully applied for selective adsorption of Hg2+ ions in the presence of Cd2+ and Pb2+ ions. The characteristics of ATT@PDA-COP were determined by Fourier transform infrared spectroscopy (FT-IR), powder X-ray diffraction (PXRD), field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectrometer (EDX), CHNS elemental analysis and Brunauer-Emmett-Teller (BET) analysis. The results showed that the ATT, as an environmentally friendly agent, has been successfully grafted on the PDA-COP. The adsorption characteristics of the ATT@PDA-COP for removal of Hg2+ as a function of initial solution pH, adsorbent mass, temperature, initial Hg2+ concentration, and contact time were evaluated. The adsorption characteristics showed better conformance with the Langmuir isotherm model with a correlation coefficient (R2) value of 0.998. The maximum adsorption capacity was 312.5 mg g−1. The pH value of 6 and the contact time of 180 min were selected as optimum experimental variables. The pseudo-second-order kinetic equation could be better than other kinetic equations to describe the adsorption kinetics for Hg2+. The reusability experiments demonstrated that ATT@PDA-COP-3 can be effectively used up to at least 4 cycles.