A new quinary oxysulfide LaPbBiS3O was designed and successfully synthesized via a solid-state reaction in a sealed evacuated quartz tube. This material, composed of stacked NaCl-like [M4S6] (where M = Pb, Bi) layers and fluorite-type [La2O2] layers, crystallizes in the tetragonal space group P4/nmm with a = 4.0982(1) Å, c = 19.7754(6) Å, and Z = 2. Electrical resistivity and Hall effect measurements demonstrate that it is a narrow gap semiconductor with an activation energy of ∼17 meV. The thermopower and the figure of merit at room temperature were measured to be -52 μV/K and 0.23, respectively, which makes LaPbBiS3O and its derivatives be promising for thermoelectric applications.