The simultaneous voltammetric determination of a ternary mixture containing antihypertensive drugs amlodipine (AML), hydrochlorothiazide (HCTZ) and valsartan (VAL) by square-wave voltammetry using a cathodically pretreated boron-doped diamond electrode is reported for the first time. Three very well-resolved and reproducible oxidative processes of AML, HCTZ and VAL were found at 0.781 V, 1.14 V and 1.41 V, respectively, in Britton–Robinson buffer solution (pH 5.0). The linearity ranges were 4.9 × 10⁻⁷–7.2 × 10⁻⁶ mol L⁻¹, 2.9 × 10⁻⁶–4.5 × 10⁻⁵ mol L⁻¹, and 9.7 × 10⁻⁶–1.3 × 10⁻⁴ mol L⁻¹ for AML, HCTZ and VAL, with detection limits of 2.3 × 10⁻⁷ mol L⁻¹, 7.5 × 10⁻⁷ mol L⁻¹, and 6.2 × 10⁻⁶ mol L⁻¹, respectively. The proposed method was successfully applied to the simultaneous analysis of these antihypertensive drugs in their combined pharmaceutical tablets available in the market.