Rashba spin-orbit coupling (SOC) plays a vital role in transporting carriers under external electric field or strain in two-dimensional transition metal di-chalcogenides in the absence of inversion symmetry. Here, the effect of SOC on the electronic and thermoelectric properties of two-dimensional monolayer HfS2, HfSe2 and their Janus monolayer HfSSe has been investigated using density functional theory (DFT) and Boltzmann transport equation (BTE). A considerable enhancement in thermoelectric power factor (PF) for n-type carriers has been observed due to the SOC in all the monolayers. This enhanced power factor for n-type carriers is mainly because of increased valley degeneracy at the conduction band minima (CBM), as there is no enhancement in electron relaxation time (τ) with SOC. The lattice thermal conductivity (κph) at room temperature is very low for these monolayers compared to extensively used TMDCs such as MoS2 and WS2. This is due to the strong coupling between acoustic and optical modes, which results in low group velocity and a short lifetime of phonons in HfS2, HfSe2 and HfSSe Janus monolayers. The ultralow value of κph results a very high thermoelectric figure of merit (ZT) at room temperature for n-type carriers and which increases significantly with SOC. The highest ZT values of 0.90 (HfSe2), 0.84 (HfS2) and 0.81 (HfSSe) for n-type at 600 K has been achieved with SOC. Our theoretical investigation predicts that there is a significant effect of SOC on the thermoelectric performance of HfX2 (X = S, Se) and its Janus monolayer HfSSe and they can be a revolutionary candidate for the fabrication of a highly efficient thermoelectric power generator. © 2021 Elsevier B.V.