Ferrous ions are the major Fe species generated from sacrificial iron anodes, but ferric ions are the more active ingredients for the removal of organic matters in the acidic pH region. Therefore, a novel high-pressure electrocoagulation (HPEC) system was devised to facilitate the Fe(II)-to-Fe(III) conversion under acidic pH conditions. The effects of operation pressure (0–3 bar) and initial pH value (3–10) on the removal of color and organics from synthetic dye wastewater containing Eriochrome Black T (EBT) were investigated. The operation costs of the HPEC were compared with that of the conventional electrocoagulation (CEC) processes. The HPEC outperformed the CEC, achieving complete color removal after a short reaction time. In the initial pH range of <8, the HPEC achieved substantial color (∼100%) and organic carbon (∼78%) removals. However, a sharp drop in the removal efficiency occurred at the pH of >8 owing to the formation of Fe(III)-EBT complexes and the ineffective removal of these complexes by coagulation or adsorption. The high removal efficiencies attained under acidic and neutral pH conditions were attributed to the effective Fe(II)-to-Fe(III) conversion by dissolved oxygen under pressure followed by the formation of Fe(OH)3 precipitates. The HPEC saves significant operating costs (45–70%) compared to the CEC process.