Zn4Sb3-based compounds constitute a lead-free material family with a best thermoelectric figure of merit (zT) in the midtemperature range. Unlike being a stoichiometric compound, the mutual solubility of Zn and Sb elicits rich microstructures and the structural instability of Zn4Sb3. The solubility limit and neighboring phase relations are crucial for the delicate balance between the thermoelectric performance and phase stability of Zn4Sb3. In this work, we constructed the isothermal section of the Zn-Sb-In ternary phase diagram, built the zT map near the β-Zn4Sb3 phase region, and showed that the formation of multiscale microstructures has a profound impact on the electronic and phonon transport properties and phase stability. A high-zT zone was identified near the phase boundary between the two-phase InSb + Zn5Sb4In2 and the three-phase Zn4Sb3 + InSb + Zn5Sb4In2 regions. A sample with a nominal composition of Zn3.8In0.2Sb3 exhibited an ultralow κL of 0.2 (W m-1 K-1), an enhanced PF of 1.75 (mW m-1 K-2), and a remarkable zT value of 1.8 at 698 K. These state-of-the-art thermoelectric properties were attributed to the simultaneous enhancement in phonon scattering and the carrier energy-filtering effect in a unique hierarchical microstructure, in which InSb nanoprecipitates are dispersed in Zn5Sb4In2 coarse grains, and the latter are embedded in the host matrix In-Zn4Sb3. These results opened an avenue for environmentally friendly cost-effective midtemperature thermoelectric materials.