Flexible thermoelectric generators can power wearable electronics by harvesting body heat. However, existing thermoelectric materials rarely realize high flexibility and output properties simultaneously. Here we present a facile, cost-effective, and scalable two-step impregnation method for fabricating a three-dimensional thermoelectric network with excellent elasticity and superior thermoelectric performance. The reticular construction endows this material with ultra-light weight (0.28 g cm−3), ultra-low thermal conductivity (0.04 W m−1K−1), moderate softness (0.03 MPa), and high elongation (>100%). The obtained network-based flexible thermoelectric generator achieves a pretty high output power of 4 μW cm−2, even comparable to state-of-the-art bulk-based flexible thermoelectric generators.