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Double half‐Heuslers comprising two aliovalent half‐Heuslers are promising candidates for thermoelectric materials because of their intrinsically low lattice thermal conductivity; however, poor electronic transport properties need to be overcome. Herein, the effects of Sn doping on the electronic and thermal transport properties of p‐type Ti2FeNiSb2 to enhance the thermoelectric performance via the compositional tuning route are investigated. The power factor is significantly improved owing to the synergetic effect of the increase in the density‐of‐states effective mass and the carrier concentration. In addition, the lattice thermal conductivity is slightly reduced, benefitted from intensified phonon scattering due to lattice disordering by Sn substitution at the Sb‐site. A peak figure of merit (zT) of ≈0.28 is obtained at 973 K in Ti2FeNiSb1.8Sn0.2, which is almost twice higher than that of the pristine Ti2FeNiSb2.