By using first-principles calculations combined with the nonequilibrium Green's function method and phonon Boltzmann transport equation, we systematically investigate the influence of chirality, temperature and size on the thermoelectric properties of monolayer WSe₂ nanoribbons. The results show that the armchair WSe₂ nanoribbons have much higher ZT values than zigzag WSe₂ nanoribbons. The ZT values of armchair WSe₂ nanoribbons can reach 1.4 at room temperature, which is about seven times greater than that of zigzag WSe₂ nanoribbons. We also find that the ZT values of WSe₂ nanoribbons increase first and then decrease with the increase of temperature, and reach a maximum value of 2.14 at temperature of 500 K. It is because the total thermal conductance reaches the minimum value at 500 K. Moreover, the impact of width on the thermoelectric properties in WSe₂ nanoribbons is not obvious, the overall trend of ZT value decreases lightly with the increasing temperature. This trend of ZT value originates from the almost constant power factor and growing phonon thermal conductance.