An enhancement in the dimensionless thermoelectric figure-of-merit (ZT) of an n-type half-Heusler material is reported using a nanocomposite approach. A peak ZT value of 1.0 was achieved at 600 °C–700 °C, which is about 25% higher than the previously reported highest value. The samples were made by ball-milling ingots of composition Hf0.75Zr0.25NiSn0.99Sb0.01 into nanopowders and hot-pressing the powders into dense bulk samples. The ingots were formed by arc-melting the elements. The ZT enhancement mainly comes from reduction of thermal conductivity due to increased phonon scattering at grain boundaries and crystal defects, and optimization of antimony doping.