Nanoscale defects can induce the effective modulation of carrier concentration, mobility, and phonon scattering to secure high thermoelectric performance in semiconductors. However, it is still limited to effectively controlling nanoscale defects in thermoelectric materials. Here, argon plasma bombardment is employed to introduce a large number of point defects and dislocations in microcrystalline SnSe powders, synthesized by a solvothermal method. After sintering these powders into polycrystalline bulk materials, bulk SnSe shows the ZT increasing by up to 66.7% (from 0.36 to 0.6 at 773 K). Through detailed micro/nanostructure characterizations and first-principles calculations, the underlying mechanism is elucidated for the evaluation of thermoelectric performance. This work provides a deep understanding of the mechanism of nanoscale defects in modulating thermoelectric performance and presents experimental evidence and experience for the design and synthesis of efficient thermoelectric materials, making significant contributions to future green energy technologies.