We report on a highly sensitive and remarkably selective fluorometric assay for mercury(II) ion. It is based on the formation of a complex between thymine (T) and Hg(II) to form the [Hg(T)2(H2O)2]n complex on the surface of reduced graphene oxide (rGO). This causes the fluorescence emission of rGO (peaking at 550 nm) to be quenched. Under optimal conditions, quenching is selective and fluorescence intensity is linearly related to the concentration of Hg(II) in the range between 100 and 700 nM. The limit of detection is 5 nM. In addition to its sensitivity and selectivity, the method is simple and direct.