A rapid route of synthesizing pristine CoSb at relatively low temperature was previously developed. However, filling the voids using the same procedure is not successful. We develop a new route to fabricate In-filled cobalt skutterudites with InSb nanoinclusions InCoSb-(InSb) via solid-vapor reaction between hydrothermally synthesized CoSb powder and the indium chunk. The nanocomposites are characterized using powder X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and inductively coupled plasma mass spectrometry (ICP-MS). With the success of partial filling of In into the voids and InSb nanoinclusions, the power factor of the InCoSb-(InSb) nanocomposites is significantly enhanced, and the thermal conductivity is lowered as compared with the pristine CoSb. As a result, ZT with its highest value of 1.0 is attained for the hierarchical structured InCoSb-(InSb) nanocomposite at 575 K. The attained ZT value is among the highest ever reported value at T ≤ 575 K for In-filled cobalt skutterudites.