A new voltammetric method for the determination of Δ^9-tetrahydrocannabinol (Δ^9-THC) is described. The voltammetric experiments were accomplished in N-N dimethylformamide/water (9:1, v/v), using tetrabutylammonium tetrafluoroborate (TBATFB) 0.1 mol/L as supporting electrolyte and a glassy carbon disk electrode as the working electrode. The anodic peak current was observed at 0.0 V (vs. Ag/AgCl) after a 30 s pre-concentration step under an applied potential of −1.2 V (vs. Ag/AgCl). A linear dependence of Δ^9-THC detection was obtained in the concentration range 2.4–11.3 ng/mL, with a linear correlation coefficient of 0.999 and a detection limit of 0.34 ng/mL. The voltammetric method was used to measure the content of Δ^9-THC in samples (hemp and hashish) confiscated by the police. The elimination of chemical interferences from the samples was promptly achieved through prior purification using the TLC technique, by employing methanol/water (4:1, v/v) as the mobile phase. The results showed excellent correlation with results attained by HPLC.