Here, we report a thermoelectric material, Sn-doped Cu3SbSe4, with a figure-of-merit (ZT) of unity, which is stable up to 623 K. The Cu3SbSe4 samples, individually doped with Bi, Pb and Sn, were synthesized using conventional vacuum melting followed by the spark plasma sintering. The first-principle density functional theory (DFT) based calculations suggest that these doped Cu3SbSe4 compounds have negative formation energy throughout the temperature range, suggesting their chemical/thermal stability. The current study is to support the synchronization of theoretical and experimental study and pre-screening of the dopant which ensures stability and the TE performance. Among all the dopants studied, we realized a ZT(max) similar to 1 at 623 K with Sn doping for the optimized composition Cu3Sb0.985Sn0.015Se4 owing to the favourable optimization of the electrical and thermal transport properties. This is also supported by the DFT calculations, which suggests that Sn-doping leads to a decrease in lattice thermal conductivity.