The Janus WSSe monolayer is a new 2D direct band gap material attracting attention due to its unique physical and chemical properties. The lattice thermal conductivity is calculated using the self-consistent approach as well as single-mode relaxation time approximation. We notice that acoustic modes contribute mainly to the lattice thermal conductivity. Thermoelectric properties are investigated using the Seebeck coefficient, electrical conductivity, thermal conductivity, power factor, and figure of merit for pristine WSSe monolayer. The effect of strain on thermoelectric properties is also investigated. Interestingly, the tensile strain resulted in a lowering of the lattice thermal conductivity from 25.37 to 9.90 W m−1K−1. The valley degeneracy with biaxial strain in the valence and conduction band edges enhances the power factor. Thus, strain ultimately, improves the figure of merit from 0.72 (0.73) to 1.06 (1.08) under biaxial strain for n(p) carriers at 5 × 1020 cm−3 and 7 × 1020 cm−3, respectively. The thermoelectric efficiency of biaxial stained monolayer based device improved about 29% at 1500 K. These studies provide an avenue to engineer the thermoelectric properties using strain mediated valley degeneracy as the external stimuli. © 2020 Elsevier B.V.