To expand and utilize the potential of intermetallic compounds, the importance of quaternary half-Heusler (HH) compounds has attracted attention in the field of thermoelectrics and other inorganic materials research. In this study, triple half-Heusler (THH) Mg₂VNi₃Sb₃ was successfully synthesized by following an unconventional valence balance strategy. X-ray diffraction (XRD) and scanning electron microscope energy dispersive spectroscopy (SEM-EDS) showed the pure and homogeneous nature of samples. Even though both ternary components of Mg₂VNi₃Sb₃, i.e., MgNiSb and VNiSb are expected to be unstable with the metallic transport properties, Mg₂VNi₃Sb₃ possesses semiconducting behavior with Seebeck coefficient peak larger than 115 μV K⁻¹ around 500 K with the small bandgap of 0.12 eV. The lattice thermal conductivity (κ_L = 2.0 W m⁻¹ K⁻¹) of Mg₂VNi₃Sb₃ is much smaller than κ_L of conventional half-Heusler and double half-Heusler materials. Since high thermal conductivity has been always a problem for HH materials, the synthesis of THH with low lattice thermal conductivity can be a new direction for developing high-performance HH compounds. Through the synthesis of THH Mg₂VNi₃Sb₃, a new strategy to explore the huge compositional space for further improvement of thermoelectric performance and extended tunability of intermetallic compounds was demonstrated.