Undoped InAs and Si-doped InAs nanowires with stacking faults and twins were synthesized by catalyst-free molecular beam epitaxy and their thermoelectric enhancements due to planar defects were experimentally and theoretically demonstrated. The Seebeck coefficients, electrical resistivities, and thermal conductivities of the Si-doped and undoped InAs nanowires were measured using a micro-fabricated thermoelectric measurement platform over the temperature range of 50 to 300 K. The Si-doping increased electrical conductivity from 1.0 × 10⁻⁴ to 7.8 × 10⁻⁴ S m⁻¹, due to the increase in carrier concentration from 2 × 10¹⁷ to 8 × 10¹⁷ cm⁻³, and then decreased the thermopower from −216 to −81 μV K⁻¹ at 300 K, in agreement with the two-band model based on the Boltzman transport theory. Phonon scattering, caused by planar defects such as surfaces, twins, and stacking-fault boundaries, suppressed the lattice thermal conductivity below 3 W m⁻¹ K⁻¹ following the Callaway model. The planar defect-induced phonon scattering as well as the optimization of carrier concentration is very effective at enhancing the thermoelectric properties of InAs nanowires and is expected to be utilized for improving the thermoelectric properties of other thermoelectric materials.