An electrochemical sensor was prepared using a glassy carbon electrode modified with Si-doped nano-TiO₂ (TS/GC) for the detection of tryptophan (Trp). The electrochemical behavior of Trp at the TS/GC electrode was investigated by cyclic voltammetry (CV). It was found that the anodic peak current of Trp oxidation at the TS/GC electrode was higher than that at the bare electrode or TiO₂ nanoparticles modified GC (T/GC) electrode. This finding suggested the enhanced electrochemical properties of the TS/GC electrode, and the improvement of the electrochemical properties was attributed to the presence of oxygen functional groups on the surface of the TS and the easy transfer of electrons and holes. Thereby, the TS/GC electrode was employed to determine Trp and the effects of scan rate, pH and interferents on the response of Trp oxidation were examined. Under the optimum experimental conditions the TS/GC electrode displayed a good analytical performance. The oxidation peak current of Trp showed a linear relationship with concentration over the range of 1.0 × 10⁻⁶ to 4.0 × 10⁻⁴ M, and the detection limit was estimated to be 5.0 × 10⁻⁷ M (S/N = 3). The proposed method was simple and exhibited favorable resistance against interferences. Moreover, the electrochemical sensor was successfully employed to determine Trp in pharmaceutical samples.