In recent years, research on thermoelectric (TE) materials has intensified—thanks to the exciting potential of low-dimensional structures such as nanowires. Experiments have shown that nano- structuring materials can greatly reduce their thermal transport properties, significantly enhancing thermoelectric performance. With reduced thermal conductivity, nano-structured silicon—which is plentiful and low-cost—becomes a competitive TE material, but still trails traditional TE materials in overall performance. In this study, we show that the creation of extended defects within the crys- tal structure of silicon nanowires can create an additional enhancement. Relative to regular silicon nanowires, extended defects lead to an increased Seebeck coefficient. The effect is a consequence of the creation of dislocations and dislocation-loops, intentionally introduced in the nanowires. These defects create nano-scale potential barriers which theoretical studies have predicted can enhance silicon’s thermopower by energy filtering of low-energy carriers. Although the defects slightly reduce carrier mobility—increasing electrical resistivity in the nanowires—their presence creates an overall two-fold enhancement in the thermoelectric power factor.