Single crystals of the new pentanary thiophosphate, $K_{0.5}Ag_{0.5}Nb_2PS_{10}$ has been prepared through reactions of the elements with halide fluxes. The structure of $K_{0.5}Ag_{0.5}Nb_2PS_{10}$ has been analyzed by single crystal X-ray diffraction technique. The structure of $K_{0.5}Ag_{0.5}Nb_2PS_{10}$ is made up of one-dimensional ${^{infty}_1}[Nb_2PS_{10}]$ chains along the [001] direction and these chains are separated from one another by $Ag^+$ and disordered $K^+$ ions. This chain is basically built up from bicapped trigonal prismatic [Nb2S12] units and tetrahedral [PS4] groups. The [$Nb_2S_{12}$] units are connected together to form the linear chain, ${^{infty}_1}[Nb_2S_9]$ by sharing the S-S prism edge. Short (2.885(2) $AA$ or 2.888(2) $AA$) and long (3.743(1) $AA$) Nb-Nb distances are alternating along the chain, and the $S_2{^2-}];and;S^{2-}$anionic species are observed. The distorted octahedral coordination around the $Ag^+$ ion can be described as [2+4] bonding interaction.