[en] The authors describe a voltammetric immunosensor for morphine (MOR) that is based on a graphene screen printed electrode (GSPE) modified with gold nanoparticles (AuNP) which were electrodeposited on its surface. The gold nanoparticles were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy and cyclic voltammetry. Cysteamine was then self-assembled on the gold-modified electrodes via thiol interaction in order to introduce terminal amino groups to the electrode surface. The electrodes were then used to fabricate the immunosensor by covalent immobilization of antibodies against MOR. The sensor works on the basis of a competition between MOR and the morphine-bovine serum albumin (MOR-BSA) conjugate for the immobilized antibodies on the sensor surface and the resulting change in the square wave voltammetry reduction current using the hexacyanoferrate system as an electrochemical probe. The competitive immunoassay enables sensitive and selective detection of MOR in the 0.1 to 100 ng·mL⁻¹ MOR concentration range, with a 90 pg·mL⁻¹ detection limit. The method was applied to the determination of MOR in spiked saliva samples and showed high recoveries. We believe that this immunosensor offers a simple, rapid, sensitive and accurate MOR test. < Image>.