Inorganic plastic semiconductors play a crucial role in the realm of flexible electronics. In this study, we present a cost-effective plastic thermoelectric semimetal magnesium bismuthide (α-MgBi), exhibiting remarkable thermoelectric performance. Bulk single-crystalline α-MgBi exhibits considerable plastic deformation at room temperature, allowing for the fabrication of intricate shapes such as the letters “SUSTECH” and a flexible chain. Transmission electron microscopy, time-of-flight neutron diffraction, and chemical bonding theoretic analyses elucidate that the plasticity of α-MgBi stems from the helical dislocation-driven interlayer slip, small-sized Mg atoms induced weak interlayer Mg-Bi bonds, and low modulus of intralayer MgBi networks. Moreover, we achieve a power factor value of up to 26.2 µW cm K along the c-axis at room temperature in an n-type α-MgBi crystal. Our out-of-plane flexible thermoelectric generator exhibit a normalized power density of 8.1 μW cm K with a temperature difference of 7.3 K. This high-performance plastic thermoelectric semimetal promises to advance the field of flexible and deformable electronics.