Mercury pollution is a global problem, and the development of stable and sensitive fluorescent probes for mercury ions in the water phase has long been sought. In this work, a novel fluorescence resonance energy transfer (FRET)-based ratiometric sensor for detecting Hg2+ in pure water was demonstrated. Polymeric nanoparticles prepared by miniemulsion polymerization of methyl methacrylate and acrylic acid were used as the scaffold for the FRET-based sensor. A, hydrophobic fluorescent dye nitrobenzoxadiazolyl derivative, (NBD) was embedded in the nanoparticles during the polymerization and used as the donor. A spirolactam rhodamine derivative SRHB-NH2 was synthesized and then covalently linked onto the particle surface and used as an ion recognition element. The presence of Hg2+ in the water dispersion of nanoparticles induced the ring-opening reaction of the spirolactam rhodamine moieties and led to the occurrence of the FRET process, affording the nanoparticle system a ratiometric sensor for Hg2+. The nanoparticle sensor can selectively detect the Hg2+ in water with the detection limit of 100 nM (ca. 20 ppb). It has been found that the FRET-based system with smaller nanoparticles as the scaffold exhibited higher energy transfer efficiency and was more preferred for the accurate ratiometric detection. Moreover, the FRET-based sensor was applicable in a relatively wide pH range (pH 4-8) in water; thus, this approach may provide a new strategy for ratiometric detection of analytes in environmental and biological applications.