On-demand transient electronics, technologies referring subsequent material disintegration under well-defined triggering events and programmed timelines, offer exceptional clinical experiences in diagnosis, treatment, and rehabilitation. Despite potential benefits, such as the elimination of surgical device removal and reduction of long-term inimical effects, their use is limited by the non-transient conventional power supplies. Here, we report an ultrasound-mediated transient triboelectric nanogenerator where ultrasound determines the energy generation and degradation period. Our findings on finite element method simulation show that the porous structures of the poly(3-hydroxybutyrate-co-3-hydroxyvalerate) enables opposing transient behaviors under different ultrasound intensity, 0.5 W/cm² and 3.0 W/cm². Besides, the addition of polyethylene glycol improves triboelectric output performance; the voltage output increased by 58.5 %, from 2.625 V to 4.160 V. We successfully demonstrate the tunable transient performances by ex-vivo experiment using a porcine tissue. This study provides insight into practical use of implantable triboelectric nanogenerators based on ultrasound-triggered transient material design.