MEMS-based piezoelectric ultrasonic energy harvesters (PUEH) have become one of the most promising options for replacing or transferring energy to batteries in medical implants, where device miniaturization and power optimization are needed. Among the most commonly used piezoelectric materials in PUEH, lead zirconate titanate (PZT) is widely acknowledged for its excellent piezoelectric properties, good stability, and low cost. However, the performance of PZT degrades when the processing temperature approaches and exceeds half of its Curie temperature Tc, limiting its application. Here, we demonstrate a highly miniaturized, low-temperature fabricated MEMS-based PUEH with an effective ultrasound harvesting area of 0.79 mm2 and an effective device volume of 0.35 mm3. The low-temperature adhesive epoxy bonding ensures the temperature throughout the entire fabrication process remains below 85°C, which preserves the properties of the integrated piezoelectric material to the greatest extent. This allows the use of bulk PZT-5H, a material that possesses superior piezoelectric properties, but has a relatively low Tc, to enhance device performance. Our device outputs a root-mean-square (RMS) voltage of 0.62 V and an RMS power of 0.19 mW on a 2 kΩ resistive load at an optimum operating frequency of 200 kHz, with a reception distance of 20 mm in water and input acoustic power intensity of 178 mW/cm2. The proposed design and fabrication technique enable our device to achieve the smallest effective size among the reported MEMS-based PUEH while still being capable of powering up numerous implantable medical devices and being compatible with various commercially available power management units. [2024-0081]