The effects of S doping at the oxygen site on the thermoelectric properties of BiOCuSe have been investigated. The partial substitution of S ions at the O sites of BiOCuSe was achieved by sulfurization using CS2 gas. Analysis of the powder X-ray diffraction data indicates that the ZrCuSiAs structure of BiOCuSe is retained after sulfurization. Substitution of O with S leads to an increase in the lattice parameters and a decrease in the band gap. The electrical conductivity rises due to the increase of the electronic contribution with doping. S-doped BiOCuSe materials behave as a p-type semiconductor. The thermoelectric properties of S-doped BiOCuSe materials can be understood through the analysis of the electronic band structure and the density of states close to the Fermi level. The substitution of O sites with S provides possible directions toward the enhancement of the thermoelectric figure of merit of oxide materials with low electrical conductivity.