Biodegradable transient devices is an emerging type of electronics that could play an essential role in medical therapeutic/diagnostic processes, such as wound healing, tissue regeneration, etc. The associated biodegradable power sources, however, remain as a major challenge towards future clinical applications, as the demonstrated electrical stimulation and sensing functions are limited by wired external power or wireless energy harvesters via near-field coupling. We propose novel materials strategies and fabrication schemes that enable a high-performance fully biodegradable magnesium-molybdenum trioxide battery as an alternative approach for in vivo on-board power supply. The battery can deliver a stable high output voltage as well as prolonged lifetime that could satisfy requirements of representative implantable electronics. The battery is fully biodegradable, and demonstrates desirable biocompatibility. The battery system provides a promising solution to advanced energy harvesters for self-powered transient bioresorbable implants as well as eco-friendly electronics.