Nanoengineering is an effective strategy to strengthen phonon scattering, reduce lattice thermal conductivity and boost thermoelectric material performance. However, nanostructure features are generally in the size of ∼10 nm, and the fine control of nanostructure characteristics down to the sub-nano level (below several nanometers) remains as a key challenge. Here, we demonstrate that solvothermally synthesized Bi₂Te₃ can re-crystallize preferentially at the grain boundaries to form sub-nano boundary regions with width <2 nm via the optimization of sintering conditions. The optimized formation process of these sub-nano boundary regions can induce synergistic effects, including strengthened mid- to short-wavelength phonon scattering, weakened lattice/carrier scattering, carrier concentration optimization, weakened band degeneracy, as well as the optimized bipolar effect. Finally, a wide plateau figure of merit (zT) of >1.2 (from ∼323 to ∼423 K) and a high average zT of ∼1.18 (from 303 to 473 K) have been achieved in the Bi₂Te₃ pellet sintered at 593 K. This study not only reveals the formation mechanism of sub-nano boundary regions but also demonstrates that these sub-nano boundary regions and their formation process can effectively induce synergistic effects contributing to high thermoelectric performance, and guide the design of high-performance thermoelectric materials.