Pollution monitoring in waterways and oceans is often performed in a laboratory on samples previously taken from the environment. The integration of molecular imprinting polymer nanoparticles (MIP-NPs) with a novel, to the best of our knowledge, fiber optic interferometer allowed a fast and selective detection of water pollutant 2,4-Dichlorophenol (2,4-DCP). The proposed sensor with an increased surface-to-volume ratio of MIP-NPs provided an enhanced sensitivity of 17.1 nm/µM and a wide operating range of 0.1-100 µM. It showed a highly repeatable performance and potential to measure up to nM concentrations. This integrated technique is suitable for the development of compact, stable, precise, and sensitive biosensors for online monitoring and remote chemical sensing applications.