We present the synthesis and characterization of single crystals and polycrystalline samples of Ba2Cu2.1(1)Ag1.9(1)Se5. A single-crystal X-ray diffraction study confirms its monoclinic structure (space group: C2/m) with lattice parameters of a = 16.0342(15) Å, b = 4.4162(4) Å, c = 9.1279(9) Å, β = 124.005(2)°, V = 535.82(9) Å3, and Z = 2. The structure of Ba2Cu2.1(1)Ag1.9(1)Se5 consists of polyanionic [∞2Cu2.1(1)Ag1.9(1)Se5]4− layers. The negative charges on these layers are counterbalanced by the filling of Ba2+ cations in the structure. The Ag and Cu atoms are statistically disordered at the same sites. The Ag/Cu atoms are bonded with four Se atoms in a distorted tetrahedral fashion. The structure also contains linear Se34− units with intermediate Se⋯Se interaction of 2.7734(14) Å. The Ba2Cu2.1(1)Ag1.9(1)Se5 can be charge-balanced as (Ba2+)2(Cu+)2.1(Ag+)1.9(Se2−)2(Se34−)1. The optical absorption study performed on a polycrystalline sample with a loaded composition of Ba2Cu2.1Ag1.9Se5 reveals a direct bandgap of 1.0(2) eV, and the indirect bandgap was found to be below 0.5 eV. The resistivity study also confirmed the semiconducting nature of the sample. The thermoelectric properties, including thermal conductivity and Seebeck coefficient, have been studied as a function of temperature. The polycrystalline Ba2Cu2.1Ag1.9Se5 has a very low thermal conductivity value of 0.46 W/mK at 673 K. The positive sign of the Seebeck coefficient values indicates the polycrystalline Ba2Cu2.1Ag1.9Se5 is a p-type semiconductor with holes as the majority of charge carriers. Further, the charge storage behavior of the material was investigated for supercapacitor application. The material delivers 76 F g−1 at 0.5 A g−1 current density in 1 M KOH electrolyte and is stable for 10,000 cycles retaining 85% capacitance. © 2023 Elsevier Masson SAS