The fast and accurate real-time monitoring of hydrogen peroxide (H₂O₂) secreted from living cells plays a critical role in clinical diagnosis and management. Herein, we report low-cost and self-supported MoS₂ nanosheet arrays for non-enzymatic eletrochemical H₂O₂ detection. Under the optimal test conditions, such MoS₂ electrodes exhibit extremely promising electrocatalytic performance with a low detection limit of 1.0 μM (S/N = 3) and an excellent sensitivity of 5.3 mA mM⁻¹ cm⁻². Furthermore, the detection of the trace amount of H₂O₂ secreted from live A549 cancer cells was successfully performed with this biosensor.