Entropy engineering is a crucial strategy for improving the performance of thermoelectric materials; however, achieving a single-phase solid solution becomes increasingly difficult when multiple elemental constituents are introduced into the matrix material. In this work, we propose an innovative approach that integrates the high solid solution solubility of compounds with similar chemical bonding characteristics, specifically, metavalent bonds, with the concept of entropy engineering. Through the co-alloying GeTe, PbTe, AgBiTe<inf>2</inf> and SnTe, we synthesized the medium-entropy alloy Sn<inf>9</inf>AgBiGe<inf>3</inf>Pb<inf>3</inf>Te<inf>17</inf>. Band convergence promoted by entropy alloying significantly enhances the Seebeck coefficient. Meanwhile, as the configurational entropy parameter increases, the increased disorder leads to the formation of defects that enhance phonon scattering, significantly suppressing the lattice thermal conductivity. Consequently, Sn<inf>9</inf>AgBiGe<inf>3</inf>Pb<inf>3</inf>Te<inf>17</inf> exhibited a peak zT value of 1.32 at 823 K, with an average zT value of 0.85 across the 300 K–823 K temperature range. Calculations using Snyder's model suggest a conversion efficiency of 11.68 %. Moreover, a record-high Vickers hardness of 171.51 H<inf>v</inf> was achieved. These results not only demonstrate that entropy engineering offers a powerful approach to improve the thermoelectric properties of SnTe-based systems but also offer methodological guidance for the selection of alloying elements in other metavalently bonded thermoelectric systems.