Half-Heusler alloys provide a versatile platform for tuning electronic and thermal transport through chemical substitution. Here, we investigate the thermoelectric and magnetotransport properties of spark-plasma-sintered YNi₀.5Pt₀.5Bi. Partial substitution of Ni by Pt drives YNiBi toward semimetallic transport, characterized by hole-dominated conduction, a low Hall carrier concentration of 1.9 × 10¹⁹ cm⁻³, and a sizeable nonsaturating magnetoresistance of 18% at 300 K under 14 T. Compared with YNiBi, Pt alloying increases the room-temperature Seebeck coefficient from approximately 90 to 130 μ V K⁻¹, while reducing the thermal conductivity to 3.15 W m⁻¹ K⁻¹. Single-parabolic-band analysis indicates an enhanced carrier effective mass, resulting in a room-temperature power factor of 13.7 μ W cm⁻¹ K⁻² and a figure of merit (zT= 0.13). First-principles calculations further show a Pt-induced closing of the electronic band gap and a low density of states near the Fermi level, consistent with the experimentally observed semimetal-like transport. These results demonstrate that Ni/Pt alloying provides an effective route for simultaneously tuning the electronic, magnetotransport, and thermoelectric properties of YNiBi-based half-Heusler compounds.