Thallium (Tl) is a highly toxic element whose occurrence is widespread in soil and groundwater. The present study investigates the oxidation of Tl(I) with boron-doped diamond (BDD) anode in an electrochemical system, compared with PbO2 and carbon felt materials. Under initial conditions of Tl(I) of 10 mg L−1, pH of 2.0, and current density of 5 mA cm−2, 99.2 ± 0.9% of Tl(I) is oxidized to Tl(III) within 15 min. This process is suppressed by the increase of initial Tl(I) concentration and initial pH, respectively, while it is enhanced with the increase of current density. Compared with direct electrochemical effect, indirect electrochemical oxidation with the generated oxidants mainly contribute to the excellent performance and ·OH plays a significant role. Subsequent coagulation/precipitation realizes the nearly complete removal of total Tl in the exhaust electrolyte and the quality of the effluent can meet EPA drinking water standards. Analysis of the generated precipitate further indicates that Tl(III) is the main oxidation state of Tl. This study offers a potentially attractive method for remediation of Tl-polluted groundwater.