A boron-doped diamond (BDD) electrode was used for the electroanalytical determination of estriol hormone in a pharmaceutical product and a urine sample taken during pregnancy by square-wave voltammetry. The optimized experimental conditions were: (1) a supporting electrolyte solution of NaOH at a pH of 12.0, and (2) a frequency of 20Hz, a pulse height of 30mV and a scan increment of 2mV (for the square-wave parameters). The analytical curve was linear in the concentration range of 2.0×10⁻⁷ to 2.0×10⁻⁵molL⁻¹ (r=0.9994), with a detection limit of 1.7×10⁻⁷molL⁻¹ and quantification limit of 8.5×10⁻⁷molL⁻¹. Recoveries of estriol were in the range of 98.6–101.0%, for the pharmaceutical sample, and 100.2–103.4% for the urine sample, indicating no significant matrix interference effects on the analytical results. The accuracy of the electroanalytical methodology proposed was compared to that of the radioimmunoassay method. The values for the relative error between the proposed and standard methods were −7.29% for the determination of estriol in the commercial product and −4.98% in a urine sample taken during pregnancy. The results obtained suggest a reliable and interesting alternative method for electroanalytical determination of estriol in pharmaceutical products and urine samples taken during pregnancy using a boron-doped diamond electrode.