A highly sensitive electrochemical sensor was prepared for the determination of L-cysteine using a modified multiwall carbon nanotubes paste electrode in the presence of 3,4-dihydroxycinnamic acid (3,4-DHCA) as a mediator, based on an electrocatalytic process. The results indicate that the elec-trode is electrocatalytically efficient for the oxidation of L-cysteine in the presence of 3,4-DHCA. The interaction between the mediator and L-cysteine can be used for its sensitive and selective deter-mination. Using chronoamperometry, the catalytic reaction rate constant was calculated to be 2.37 × 102 mol-1 L s-1. The catalytic peak current was linearly dependent on the L-cysteine concentration in the range of 0.4-115 μmol/L. The detection limit obtained by linear sweep voltammetry was 0.25 μmol/L. Finally, the modified electrode was examined as a selective, simple, and precise new elec-trochemical sensor for the determination of L-cysteine in real samples.