We report on the circuit design and experimental demonstration of a battery-less, self-powering temperature sensor node, powered entirely by a piezoelectric (PZE) ambient vibrational energy harvester, and enabled by a novel ultra-low-power application-specific integrated circuit (ASIC). The ASIC collects and stores energy generated by the PZE transducer into a supercapacitor, powers both the on-ASIC temperature sensor and the on-board RF module for wireless temperature sensing applications. The ASIC is fabricated using a standard 0.5 μm CMOS process. The quiescent current in the entire ASIC is only 55 nA for energy harvesting, and 255 nA when temperature sensing is further included. Such self-powering temperature sensors can help enable low-cost transformation of today's ordinary buildings into energy-efficient smart buildings.