The first example of alkali/transition metal thioaluminates KCuAlS was synthesized and characterized. The compound crystallizes in the orthorhombic space group Pnna (no. 52), with unit cell parameters a = 5.471 (6) Å b = 5.468 (6) Å and c = 13.111 (1) Å within a pseudo-tetragonal lattice. Its crystal structure consists of 2D [CuAlS] polyanionic anti-PbO type layers, in which Al and Cu atoms share the atomic positions, separated by K cations. The (Cu/Al) atomic positions are tetrahedrally coordinated by S atoms, whereas K atoms are surrounded by eight S atoms in a cubic fashion. The coordination environments of the Al and K cations were confirmed by solid-state Al and K NMR, respectively. The title compound is a semiconductor with a direct bandgap of 2.96 eV, and thermally stable up to 550 °C.