The article reports a synthetic strategy to grow Janus monolayers of transition metal dichalcogenides, specifically MoSeS, by breaking the out-of-plane structural symmetry. The authors start with a MoS2 monolayer and replace the top-layer sulfur atoms with hydrogen atoms using a remote hydrogen plasma. Subsequently, the hydrogen atoms are replaced with selenium atoms through thermal selenization, forming a structurally stable Janus MoSeS monolayer. The Janus structure is confirmed using scanning transmission electron microscopy and energy-dependent X-ray photoelectron spectroscopy. The optical gap of the Janus MoSeS monolayer is 1.68 eV, close to the average of MoS2 and MoSe2. The out-of-plane dipole in the monolayer is probed using second harmonic generation, and the piezoelectric response is measured using piezoresponse force microscopy. The study highlights the potential of these asymmetric monolayers in optoelectronics and spintronics.The article reports a synthetic strategy to grow Janus monolayers of transition metal dichalcogenides, specifically MoSeS, by breaking the out-of-plane structural symmetry. The authors start with a MoS2 monolayer and replace the top-layer sulfur atoms with hydrogen atoms using a remote hydrogen plasma. Subsequently, the hydrogen atoms are replaced with selenium atoms through thermal selenization, forming a structurally stable Janus MoSeS monolayer. The Janus structure is confirmed using scanning transmission electron microscopy and energy-dependent X-ray photoelectron spectroscopy. The optical gap of the Janus MoSeS monolayer is 1.68 eV, close to the average of MoS2 and MoSe2. The out-of-plane dipole in the monolayer is probed using second harmonic generation, and the piezoelectric response is measured using piezoresponse force microscopy. The study highlights the potential of these asymmetric monolayers in optoelectronics and spintronics.