A nanocomposite of graphene oxide (GO), yttrium oxide (Y2O3) nanoparticles, and Nafion was prepared and used to modify a glassy carbon electrode (GCE) by a simple drop casting technique. The modified electrode sensor was then used for the electrochemical detection of l -3,4-dihydroxyphenylalanine (l -DOPA). Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were used for the characterization of the modified electrode. Square wave voltammetry (SWV) and cyclic voltammetry (CV) were also employed for electrochemical detection of l -DOPA and characterization of the modified electrode. The effect of SWV parameters, ratio of GO:Y2O3 in the composite, and the electrolyte solution pH on the oxidation peak currents were examined to optimize experimental conditions. The results of the optimization steps revealed the best experimental parameters based on the current response for oxidation of l -DOPA were 55 mV for pulse amplitude, 10 mV for step potential, 30 Hz for frequency, 2:1 for the GO:Y2O3 ratio, and 3.5 for pH. Under these conditions, the GCE modified with GO-Y2O3 showed a linear current response for oxidation of l -DOPA in the range of 0.5–350 μM with a limit of detection (LOD) of 0.05 μM, a limit of quantification (LOQ) of 0.17 μM, and a heterogeneous rate constant (k°) of 1.07 × 10−2 cm s−1. Increased sensitivity compared to a GO modified electrode was clearly observed. The electrode also exhibited selectivity towards l -DOPA in the presence of common interferences such as ascorbic and uric acid, and excellent stability and reproducibility over time. Finally, the modified electrode was utilized for the detection of l -DOPA in biological samples.