This study aims to use graphene quantum dots (GQDs) as a fluorescence switching sensor (turn on-off) for the simultaneous detection of cyanide (CN) and ferricyanide [Fe(CN)] in wastewater samples. The GQDs were synthesized by pyrolyzing solid citric acid. The intrinsic blue color of the solution was observed under ultraviolet irradiation. The fluorescence spectrum was maximized at both excitation and emission wavelengths of 370 and 460 nm, respectively. The fluorescence intensity of GQDs decorated with Hg (turn-off mode as the starting baseline) could be selectively turned on in the presence of CN and once back to turn-off mode by [Fe(CN)]. The fluorescence switching properties were used to develop a fluorescence turn-on-off sensor that could be used to detect trace amounts of CN and [Fe(CN)] in water samples. For highly sensitive detection under optimum conditions (Britton-Robinson buffer solution in the pH range of 8.0-9.0, linearity ranges of 5.0-15.0 μM ( = 0.9976) and 10.0-50.0 μM ( = 0.9994), respectively, and detection limits of 3.10 and 9.48 μM, respectively), good recoveries in the ranges of 85.89-112.66% and 84.88-113.92% for CN and [Fe(CN)], respectively, were recorded. The developed methods were successfully used for the simultaneous and selective detection of CN and [Fe(CN)] in wastewater samples obtained from local municipal water reservoirs.