Single crystals of a new selenide, Y3Fe0.5SiSe7, have been synthesized by a reaction of elements at high temperatures inside a vacuum-sealed fused silica tube. The title phase crystallizes in the hexagonal noncentrosymmetric P63 space group (Pearson symbol hP23) at room temperature (RT), as established by a single-crystal X-ray diffraction study. The structure is non-stoichiometric with refined lattice parameters of a = b = 10.1971(1) Å, c = 5.9799(2) Å, and V = 538.49(2) Å3 (Z = 2). The asymmetric unit of the structure consists of six crystallographically independent atomic positions: one Y, one half-occupied Fe, one Si, and three Se sites. The building blocks of the structure are a distorted FeSe6 octahedron, YSe6 trigonal prism, and SiSe4 tetrahedron that are fused together via sharing of anions to create a three-dimensional periodic network. The charge-balanced formula of the structure can be achieved as (Y3+)3(Fe2+)0.5(Si4+)1(Se2−)7. A polycrystalline sample of Y3Fe0.5SiSe7 was also synthesized by the high-temperature reactions of stoichiometric amounts of elements. A direct bandgap value of 1.1(1) eV for the polycrystalline sample was estimated by a solid-state UV–Vis–NIR absorption spectroscopic study. A resistivity study on the polycrystalline Y3Fe0.5SiSe7 sample corroborates its semiconducting nature. The polycrystalline sample also shows an extremely low thermal conductivity value of ∼0.26 Wm−1K−1 at 773 K. The ab-initio optical and thermoelectric parameters of the title structure have been estimated from the electronic band structure. The relative bond strengths of Y, Si, and Fe atoms with Se atoms are investigated by the COHP study. © 2023 Elsevier Masson SAS