In this work, a novel and ultrasensitive sandwich-type electrochemical immunosensor was developed for the quantitative detection of prostate specific antigen (PSA), a well-known prostatic tumor biomarker. A 3D-structured reduced graphene oxide–multiwalled carbon nanotube–palladium nanoparticle (NP) nanocomposite (rGO–MWCNT–Pd) was prepared in a simple and environmentally friendly way and utilized for the adsorption of secondary antibodies (Ab₂). It is used as a novel enzyme-mimicking label to develop a sandwich-type electrochemical immunosensor for signal amplification, and shows better electrocatalytic activity for the reduction of hydrogen peroxide (H₂O₂) than rGO–CNT or rGO–Pd due to the synergetic effect of the rGO–MWCNT–Pd nanocomposite. The nanocomposite of Au NPs decorated on aminated graphene (Au–NH₂–GS) is used as a platform for the immobilization of primary antibobies (Ab₁) via an amide reaction between the –NH₂ group of the Au–NH₂–GS and the –COOH groups of Ab₁. Due to the excellent electrocatalytic activity of rGO–MWCNT–Pd for the reduction of H₂O₂, electrochemical amperometric changes at different concentrations of PSA were studied after the immunoreaction. Under the optimum experimental conditions, the proposed electrochemical sandwich-type immunosensor exhibits a low detection limit (0.17 pg mL⁻¹), and a wide linear range (from 0.5 pg mL⁻¹ to 15 ng mL⁻¹) for the detection of PSA. It also shows high sensitivity, good selectivity and stability, and has great potential in clinical and diagnostic applications.