Selective detection of toxic pollutants present in water has been a severe challenge to the scientific community for a long time. The noble integration of optical fiber-based interferometry with a bio-recognizing element molecular imprinting polymer (MIP) exhibits a promising technique for selective and susceptible biochemical detection. Here, we report a compact, stable, reproducible, and label-free optical sensor using a combined approach of photonic crystal fiber (PCF)-based modal interferometry and MIP nanoparticles (MIP-NPs) for selective detection of water pollutant p-cresol with an extremely low limit of detection (LOD). The MIP-NPs having a greater surface-to-volume aspect ratio allows more target analytes to bind. The sensor immobilized with MIP-NPs shows unprecedented sensitivity of 1.865 × 108 nm/M with specific and repeatable detection performance for a broad dynamic detection range of 10–8–10–3 M. The sensor offers a remarkable detection ability of as low as 1.55 nM concentrations of p-cresol in the aqueous medium, for water quality monitoring. Fast response, high resolution, compact size, label-free broad detection range, and selective reusable performance of the proposed sensor exhibit potential for board practical utilizations, including medical sectors, online and remote biosensing, and water resource monitoring.