In this work, a LaB6‐alloying strategy is reported to effectively boost the figure‐of‐merit (ZT) of Ge0.92Bi0.08Te‐based alloys up to ≈2.2 at 723 K, attributed to a synergy of La‐dopant induced band structuring and structural manipulation. Density‐function‐theory calculations reveal that La dopant enlarges the bandgap and converges the energy offset between the sub‐valence bands in cubic‐structured GeTe, leading to a significantly increased effective mass, which gives rise to a high Seebeck coefficient of ≈263 µV K−1 and in turn a superior power factor of ≈43 µW cm−1 K−2 at 723 K. Besides, comprehensive electron microscopy characterizations reveal that the multi‐scale phonon scattering centers, including a high density of planar defects, Boron nanoparticles in tandem with enhanced boundaries, dispersive Ge nanoprecipitates in the matrix, and massive point defects, contribute to a low lattice thermal conductivity of ≈0.67 W m−1 K−1 at 723 K. Furthermore, a high microhardness of ≈194 Hv is witnessed in the as‐designed Ge0.92Bi0.08Te(LaB6)0.04 alloy, derived from the multi‐defect‐induced strengthening. This work provides a strategy for developing high‐performance and mechanical robust middle‐temperature thermoelectric materials for practical thermoelectric applications. The LaB6‐alloying strategy is demonstrated to enhance ZT of GeTe‐based alloys up to ≈2.2 at 723 K owing to rational band structuring and structural manipulation. The dissolving of LaB6 leads to multiscale crystal imperfections, contributing to the low lattice conductivity and the high hardness.