Janus SeMoS monolayers (MLs) are synthetic 2D materials with unique electronic properties, as theory predicts, but their experimental exploration has been hindered by the low quality of the samples. Here we report a synthesis of high-quality Janus MLs on gold substrates by thermal exchange reaction taking place at the ML/Au(111) interface. The synthesized Janus SeMoS MLs were characterized by complementary techniques, and insights into the topography and electronic properties of the system were obtained. Specifically, due to the lattice mismatch with the Au(111), a moiré pattern with a periodicity of 2.9 nm was observed. A precise experimental determination of the lattice constant of Janus SeMoS of 3.22 ± 0.01 Å was obtained, and the measured spin-orbit splitting at the