We report a graphene oxide (GO)-based fluorescent sensor for Hg(2+) detection in aqueous solutions by using hybridization chain reactions (HCRs). GO is used as an adsorption material for capturing single-stranded DNA and an efficient fluorescence quencher for reducing the background signal. In the detection strategy, two hairpin probes and a helper DNA are employed. Without Hg(2+), they are adsorbed by the GO and the fluorescence of one of the hairpin probes is quenched. In the presence of Hg(2+), the HCRs between the two hairpin probes are initiated by Hg(2+) with the aid of the helper DNA through T-Hg(2+)-T coordination chemistry. The double-stranded DNA products of the HCRs are released by the GO and the fluorescence is recovered. The detection limit of the sensing method is 0.3 nM, which is sufficiently sensitive for practical applications. The sensing system also exhibits high selectivity against other divalent metal ions, and the application of the sensor for drinking water shows that the proposed method works well for real samples.