An electrochemical sensor for the sensitive and selective detection of cysteine is proposed based on a gold nanoparticle (AuNP)–iron(III) phthalocyanine (FePc) modified graphite paste electrode. The sensor was characterised using scanning electron microscopy (SEM/EDX), transmission electron microscopy (TEM), cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Cyclic voltammetry studies demonstrated that the electrochemical behaviour of cysteine at the AuNP–FePc modified graphite paste electrode is considerably improved compared to both the blank (unmodified) and FePc-modified graphite paste electrodes. The enhancement of the anodic redox signal for cysteine was due to a catalytic effect of gold nanoparticles which was investigated using three different graphite paste electrodes − 0.02, 0.055 and 0.11 wt% Au nanoparticles, each with a fixed quantity of FePc (5 wt%). The sensor fabricated using 0.055 wt% AuNPs exhibited optimum sensitivity as examined via differential pulse voltammetry measurements with an analytical range of 50–1000 μM and a LOD of 0.27 μM. The sensor was utilised for the determination of cysteine in pharmaceutical preparations.