Pyrite nanocrystals (PNCs) is a potential alternative to conventional quantum dots due to their favorable properties such as sustainability, cost-effectiveness, low toxicity and tunable luminescence. In this work, we explored peroxidase-like activity of FeS PNCs, synthesized by a simple solvothermal method. Nanozymic properties of PNCs were studied with TMB as a substrate. The reaction mechanism was evaluated by Michaelis-Menten equation and the radical scavengers. Utilizing the great enzyme-mimic activity of PNCs we were developed a sensitive colorimetric sensor for determination of ceftriaxone in the range of 0.5-20 nM, and detection limit was 0.13 nM. On the basis of clear color change in the solution, a simple but sensitive smartphone-based platform in the range 10-40 nM with the detection limit of 7.8 nM was also developed. Applicability of these sensors was studied for analysis of various water samples, demonstrating their great potential for real analytical applications.