An ultra-low-startup-voltage thermoelectric energy harvesting system assisted by a piezoelectric generator (PEG) is presented in this paper. When the energy harvesting system is implemented in a place where there is mechanical vibration, the associated PEG can generate a stable clock signal and drive the boost converter to start from the cold state even at an extremely low thermoelectric generator (TEG) voltage. The simulations show that the proposed system can start the TEG system from the cold state from as low as 10 mV of TEG voltage while keeping a 63.9% efficiency. The peak power conversion efficiency is achieved at 74.9% when the TEG voltage is 50 mV.