The thermoelectric materials so far discovered have shown thermoelectric behavior at relatively higher temperatures. Low temperature thermoelectric materials are the need of the hour and in this work we have studied the thermoelectric behavior of 2D monolayer of XI2 (X = Sn, Si) at relatively low temperature by using Density Functional Theory (DFT) along with the Boltzmann Transport equation. We have found high thermoelectric figure of merit (ZT) for SnI2 and SiI2 at 600 K. At room temperature the maximum ZT is 0.66 (0.35), 0.78 (0.51) for p-type (n-type) SnI2 and SiI2 respectively and at 600 K it is 0.83 for SiI2. The study of the optical properties of both the materials shows that both of them have indirect band gap with SnI2 having a band gap of 2.06 eV where as SiI2 has a band gap of 1.63 eV. Both the materials have a very high absorption coefficient in the ultraviolet (UV) region hence these materials can be used as high sensitive UV photodetectors. Thus the SnI2 and SiI2 2D monolayers can have potential application in optoelectronic as well as thermoelectric device fabrication. © 2020 Elsevier B.V.