Significant efforts have been devoted to developing trace-level quantification of Hg2+. In this work, a novel fluorescence detection (FD) approach combined with preconcentration of Hg2+ in ionic liquid (IL), N-octylpyridinium tetrafluoroborate ([OPy]+[BF4]-), was developed for the determination of trace Hg2+ in water. The temperature-controlled ionic liquid dispersive liquid-liquid microextraction (TC-IL-DLLME) technique was used to improve the enrichment factor. After extraction and centrifugal separation, precipitated IL was diluted with water and acetonitrile containing the fluorescent probe. Under optimum conditions, the photoluminescence intensity of the fluorescence emission peak was found to be linear with the concentration of Hg2+ in the range from 0.1 to 0.5 μg L-1. The limit of detection for the TC-IL-DLLME-FD method was calculated to be 0.0342 μg L-1 (S/N = 3). Further, the selectivity of the sensor was measured by evaluating its response to other heavy-metal ions and was shown to be excellent. Recoveries were assessed by spiking water samples with different Hg2+ standard stock solutions, giving satisfactory recoveries from 80 to 110%.