The rapid advancement of intelligent wearable electronics imposes the insurgent need for power sources that are deformable, compliant and stretchable. Power sources with these characteristics are difficult and challenging to achieve. The use of liquid metals as an electrode may provide a viable strategy to produce such power sources. Here, we would like to introduce a liquid metal-based triboelectric nanogenerator (LM-TENG) by employing Galinstan as the electrode and silicone rubber as the triboelectric and encapsulation layer. The small Young’s modulus of the liquid metal ensures the electrode to remain continuously conductive under deformations, stretchable to a strain as large as ~300%. The surface oxide layer of Galinstan effectively prevents the liquid Galinstan electrode from further oxidization and permeation into silicone rubber, yielding outstanding device stability. Operating in the single-electrode mode at 3 Hz, the LM-TENG with an area of 6 × 3 cm2 produces an open-circuit voltage of 354.5 V, short-circuit current of 15.6 μA, and average power density of 8.43 mW/m2, which represent the best performance values for any TENGs. Further, the LM-TENG maintains stable performance under various deformations, such as stretching, folding and twisting. LM-TENGs in different forms, such as bulk-shaped, bracelet-like and textile-like, are all able to harvest mechanical energy from human walking, arm shaking or hand patting to sustainably drive wearable electronic devices. Our work demonstrates a novel application of liquid metal as a stretchable electrode in fabricating stretchable power sources.