This paper reports an electrochemical sensor based on the covalent anchoring of aryldiazonium salt modified AuNPs to gold electrodes for the sensitive detection of cadmium ions (Cd2+). AuNPs modified with 4-nitrophenyl and 4-carboxylphenyl were first prepared, which were then immobilized in situ on gold electrodes to achieve Au-Ph-AuNP modified interfaces. Glutathione (GSH) was finally modified to Au-Ph-AuNP by amide bonding to achieve an Au-Ph-AuNP-GSH sensing interface, which was used to detect Cd2+ over the concentration range from 0.1 nM to 100 nM with a detection limit of 0.1 nM. This electrochemical sensor demonstrated high stability and sensitivity to Cd2+. It has potential use in an in situ anchoring AuNP strategy to fabricate portable devices for the onsite monitoring of trace amounts of heavy metals.