This study reports the development of a low-cost, inorganic polycrystalline Mg5Sb2.5Bi1.498Te0.002 thermoelectric (TE) material that demonstrates exceptional performance and superior plasticity at room temperature. The material exhibits a large strain of ~43% and a high figure of merit (zT) of ~0.72, surpassing both brittle Bi2(Te,Se)3 (strain ≤ 5%) and plastic Ag2(Te,Se,S) and organics (zT ≤ 0.4). The high plasticity of Mg5Sb2 and Mg3Bi2 is attributed to their inherent properties, with polycrystalline Mg5Sb2.5Bi1.498Te0.002 showing over 30% compressive strain. Single-crystalline Mg3Bi2 also demonstrates excellent deformability under bending, cutting, and twisting. By optimizing the Bi content, the material's room-temperature TE performance and plasticity are simultaneously enhanced. The dense dislocation network and persistent Mg-Sb/Bi bonds during slipping contribute to its high plasticity. The material's good machinability allows for easy processing into micro-scale dimensions, enabling the fabrication of both in-plane and out-of-plane flexible TE modules with promising power density. This work paves the way for significant advancements in flexible electronics and inspires further exploration of plastic inorganic semiconductors.This study reports the development of a low-cost, inorganic polycrystalline Mg5Sb2.5Bi1.498Te0.002 thermoelectric (TE) material that demonstrates exceptional performance and superior plasticity at room temperature. The material exhibits a large strain of ~43% and a high figure of merit (zT) of ~0.72, surpassing both brittle Bi2(Te,Se)3 (strain ≤ 5%) and plastic Ag2(Te,Se,S) and organics (zT ≤ 0.4). The high plasticity of Mg5Sb2 and Mg3Bi2 is attributed to their inherent properties, with polycrystalline Mg5Sb2.5Bi1.498Te0.002 showing over 30% compressive strain. Single-crystalline Mg3Bi2 also demonstrates excellent deformability under bending, cutting, and twisting. By optimizing the Bi content, the material's room-temperature TE performance and plasticity are simultaneously enhanced. The dense dislocation network and persistent Mg-Sb/Bi bonds during slipping contribute to its high plasticity. The material's good machinability allows for easy processing into micro-scale dimensions, enabling the fabrication of both in-plane and out-of-plane flexible TE modules with promising power density. This work paves the way for significant advancements in flexible electronics and inspires further exploration of plastic inorganic semiconductors.