Divalent mercuric (Hg2+) ion is one of the most prevalent forms of mercury species in waters with hightoxicity and bioaccumulation in the human body, for which sensitive and selective detection methodsare highly necessary to carry out its recognition and quantification. Here an electrochemically reducedgraphene oxide (RGO) based chemiresistive sensor was constructed and used for the detection of Hg2+ion in various water samples. Monolayer GO sheets were assembled onto interdigitated electrodes, fol-lowed by reduction through linear sweep voltammetry and then modification with a single-strandedDNA aptamer. The electrochemically derived RGO based sensor showed selective response to as lowas 0.5 nM Hg2+ion in presence of other metal ions and matrices. A comparison between chemiresistivesensors prepared with electrochemically and chemically derived RGO showed that the former had bet-ter response performance for sensing Hg2+ion. The proposed method provides a simple tool for rapid,selective and sensitive monitoring of Hg2+ion in environmental samples.