We have demonstrated a fast, extremely power-efficient, hydro-ionic microthruster that utilizes electro-osmotic propulsion for operation in a low Reynolds-number ionic environment. Powered by an onboard power supply, the microthruster achieves speeds of up to 5.24 cm/s (131 body-lengths/s) while consuming less than 252 nW/(Bodylength/s). It is 31% faster and ∼103 times more energy-efficient than prior designs. Including the onboard Al-air battery consisting of an Al anode and an Au cathode, the microthruster measures 1400μm (H) × 1400μm (W) × 400μm (L) in dimension and contains a 1000μm-wide, 300μm-long cylindrical thruster channel in the center. With significantly improved speed and drastically higher energy-efficiency, the microthruster opens up new possibilities for various self-powered biomedical applications that can operate within the power budget of modern implantable and integrable microscale batteries and bio-fuel cells.