We have built and tested a prototype miniaturized thermoelectric power source that generates 450μW of electrical power in a system volume of 4.3cm3. The measured power density of 104μWcm−3 exceeds that of any previously reported thermoelectric power system of equivalent size. This improvement was achieved by implementing a novel thermopile design in which wagon wheel-shaped thermoelectric elements contact the entire circumference of the heat source whereas traditional approaches utilize only one heat source surface. The thermopile consists of 22 wagon wheel-shaped elements (11 P–N thermocouples) fabricated from 215-μm thick bismuth–telluride wafers having ZT=0.97 at 30°C. The power source operates on a 150mW thermal input provided by an electrical resistance heater that simulates a capsule containing 0.4g of 238PuO2 located at the center of the device. Our primary research objective was to develop and demonstrate a prototype thermopile and radioisotopic thermoelectric generator (RTG) architecture with improved power density at small scales. Output power from this device, while optimized for efficiency, was not optimized for output voltage, and the maximum power was delivered at 41mV. We also discuss modifications to our prototype design that result in significantly improved voltage and power. Numerical predictions show that a power output of 1.4mW, power density of 329μWcm−3, and voltage of 362mV, is possible in the same package size.