A copper monolayer was formed on a gold electrode surface via underpotential deposition (UPD) method to construct a Cu UPD|DTBP–Protein G immunosensor for the sensitive detection of 17β-estradiol. Copper UPD monolayer can minimize the non-specific adsorption of biological molecules on the immunosensor surface and enhance the binding efficiency between immunosensor surface and thiolated Protein G. The crosslinker DTBP (Dimethyl 3,3′-dithiobispropionimidate·2HCl) has strong ability to immobilize Protein G molecules on the electrode surface and the immobilized Protein G provides an orientation-controlled binding of antibodies. A monolayer of propanethiol was firstly self-assembled on the gold electrode surface, and a copper monolayer was deposited via UPD on the propanethiol modified electrode. Propanethiol monolayer helps to stabilize the copper monolayer by pushing the formation and stripping potentials of the copper UPD monolayer outside the potential range in which copper monolayer can be damaged easily by oxygen in air. A droplet DTBP–Protein G was then applied on the modified electrode surface followed by the immobilization of estradiol antibody. Finally, a competitive immunoassay was conducted between estradiol–BSA (bovine serum albumin) conjugate and free estradiol for the limited binding sites of estradiol antibody. Square wave voltammetry (SWV) was employed to monitor the electrochemical reduction current of ferrocenemethanol and the SWV current decreased with the increase of estradiol–BSA conjugate concentration at the immunosensor surface. Calibration of immunosensors in waste water samples spiked with 17β-estradiol yielded a linear response up to ∼2200 pg mL⁻¹, a sensitivity of 3.20 μA/pg mL⁻¹ and a detection limit of 12 pg mL⁻¹. The favorable characteristics of the immunosensors such as high selectivity, sensitivity and low detection limit can be attributed to the Cu UPD|DTBP–Protein G scaffold.