The thermoelectric properties of W-substituted CrSi₂ were studied. Polycrystalline samples of hexagonal C40-structured Cr_1-xW_xSi₂ (x = 0.05, 0.10) were synthesized by arc melting and spark plasma sintering followed by heat treatment. Compared to non-substituted CrSi₂, the lattice thermal conductivity was reduced from 9.0 to 4.6 W m⁻¹ K⁻¹ by W substitution because of enhanced phonon–impurity scattering. However, the power factor was slightly reduced by W substitution, as with Mo substitution. The thermoelectric figure of merit, zT, increased with increasing W content because of the reduced thermal conductivity, reaching 0.19 at 670 K. We analyzed the observed electrical properties using a theoretical model based on the Boltzmann transport equation. The effective mass of CrSi₂ increases from 2.5 to ~3.5 by W or Mo substitution, indicating that W or Mo substitution affects the band structure of CrSi₂, which reduces the power factor.