High-temperature thermoelectric properties of tungsten-based Magne?li phase oxide (W_1-xTi_x)₁₈O₄₉ (0 ≤ x ≤ 0.25) prepared by solid state reaction followed by densification via spark plasma sintering (SPS) were studied. The Ti substitution increased the Seebeck coefficient, the power factor, and decreased both the electronic and lattice thermal conductivity. The synergistic substitution effect on the electrical and thermal properties and inherently low total thermal conductivity of 1.46 W/Km originating from the tunnel-like crystal structure of the oxide led to a significantly high ZT of 0.50 at 1073 K for the sample with x = 0.2.