Electrochemical sensors, with high accuracy, good selectivity, and linear response, have been widely used for environmental protection, health monitoring, and disease treatment. However, to date, these sensors still have limit sensitivity or otherwise require the use of high-cost materials such as noble metals and enzymes. Here, we report a novel electrochemical sensor using a topological insulator, Bi₂Te₃. Through liquid-phase exfoliation, we prepared nano- and microflakes of Bi₂Te₃ and measured their performance in hydrogen peroxide sensing via electrocatalytic reduction processes. Our devices exhibit a sensitivity of ∼4900 μA mM^–1 cm^–2 and a detection limit of ∼10^–8 molar, both of which are superior to typical noble metal-based electrochemical sensors. Through electrochemical analysis and microkinetic simulations, we extracted the kinetic parameters and gained insights into the reaction mechanism. We attribute the ultrahigh sensitivity to the facile electron transfer at the Bi₂Te₃–aqueous solution interface.