Wearable and flexible electronics warrant the development of self-powered devices to circumvent the limitations imposed by traditional energy storage devices. Triboelectric nanogenerators (TENGs) that convert waste mechanical energy from human motions into electric power offer a solution. Highly electronegative and yet conducting TENG materials that support the generation of both large potential differences and high currents are imperative for effectively harvesting electric power from human muscle movements. Here, we demonstrate that two-dimensional titanium carbide MXenes (Ti3C2Tx, where Tx stands for surface functional groups such as -O, -OH, and –F) are a new family of electrically conducting materials that are triboelectrically more negative than polytetrafluorethylene or Teflon. Specifically, flexible MXene TENGs support both high open circuit voltages ranging from ~500 to ~650 V and an instantaneous peak power up to ~0.5-0.65 mW that could power > 60 light-emitting diodes or quickly charge a 1 μF capacitor up to 50 V. Lastly, we demonstrate that flexible MXene TENGs are capable of harvesting electrical power from simple muscle movements (e.g., texting) even when the device is flexed by ~30° suggesting facile integration with wearable electronics.