Aerogels comprising poly(3-hexylthiophene) (P3HT) nanofibers dispersed in polystyrene (PS) were fabricated by freeze-drying, doped with iodine vapor, and subjected to thermoelectric property analysis, which revealed that the aerogel structure facilitated quite low thermal conductivity for temperature gradient preservation, while the P3HT nanofiber network percolating the aerogel bulk acted as an effective conductive structure. The highest Seebeck coefficient (S) of >10 mV K −1 was observed at a P3HT/PS mass ratio of 6:94. At low dopant concentrations (i.e., σ < 10 −5 S cm −1 ), the relationship between S and conductivity (σ) followed that of nondegenerate polymer semiconductors (S ∼ σ −1/4 ), while at high dopant concentrations (σ > 10 −5 S cm −1 ), the S−σ relationship followed that of metals or degenerate semiconductors (S ∼ σ −1 ), and cross-over behavior was observed. The giant Seebeck coefficient was ascribed to the unique morphology of the freeze-dried aerogel and the effect of energy filtering. A foam-based energy harvester comprising the P3HT nanofiber aerogel was demonstrated, and a significant induced voltage using a single cell indicated the advantage of P3HT nanofiber aerogels with the giant Seebeck coefficient.