Enthalpy-enhancing gas is used to optimize the ionic conductivity of atmospheric plasma-sprayed 3.9 mol % yttria-stabilized zirconia (3.9YSZ) electrolyte. In the experiment, three hydrogen gas flow rates are used to control the plasma energy. The 3.9YSZ feedstock powder is sieved to obtain particles with sizes in the range of 75–106 µm. The electrolyte has the highest hardness and lowest roughness when sprayed at hydrogen gas flow rates of 12 and 7 Lmin⁻¹, respectively. The electrolyte bulk density increases and the apparent porosity decreases with increasing hydrogen gas flow rate. A 3.9YSZ electrolyte with an ionic conductivity of 1789 µ(Ω cm)⁻¹ is obtained at 800°C with a hydrogen gas flow rate of 12 Lmin⁻¹. It is controlled by the grain-boundary conductivity. The increase in grain-boundary conductivity is closely related to a low apparent porosity, low migration energy, and a decrease in grain size. Hydrogen gas enhances the growth of columnar grains and suppresses the formation of the monoclinic phase, which is attributed to increased intragrain conductivity.