A significant amount of energy is lost in microelectronics due to resistive heating. While efficient thermal energy dissipation has been the main engineering concern, the efforts to recycle it have resulted in little success. Here, a transverse thermoelectric device is realized with a footprint <0.17 mm² and thermoelectric output >10 mV. The device combines the anomalous Nernst and spin Seebeck effects to generate a transverse voltage in response to a temperature gradient that simulates a realistic microelectronic environment. The device consists of ferrimagnetic CoTb and nonmagnetic metals in a heterostructure grown on a Silicon substrate. Tunable magnetic and electronic properties of CoTb enable significant thermoelectric conversion and external field-free operation simultaneously. It is shown that the output energy can be stored in an electrolytic capacitor. The device stability is tested over 250 charge–discharge cycles with no degradation or fatigue. These findings constitute a key step toward embedded on-chip energy harvesters that can be easily integrated into electronic architectures where significant heat is produced on the microscopic scale.