This paper presents the synthesis and characterization of a novel monolayer transition metal dichalcogenide (TMD) called Janus SMoSe. The material is synthesized by controlled sulfurization of monolayer MoSe₂, where the top layer of selenium atoms is substituted by sulfur atoms while the bottom layer remains intact. The unique S-Mo-Se tri-layer atomic structure exhibits distinct Raman vibration peaks that can be distinguished from pure MoS₂, MoSe₂, and randomly alloyed MoSₓSe₂₋ₓ. The experimental Raman peaks agree well with predictions from density functional theory (DFT) simulations. Transmission electron microscopy and time-of-flight secondary ion mass spectrometry further confirm the tri-layer structure. DFT calculations predict the bandgap of Janus SMoSe, which is consistent with experimental photoluminescence spectroscopy results. The material exhibits high basal plane hydrogen evolution reaction (HER) activity, which is attributed to the synergistic effects of intrinsic defects and structural strain in the Janus structure. The HER efficiency of Janus SMoSe is higher than that of pure MoSe₂ and MoS₂, suggesting its potential as a promising catalyst for hydrogen evolution.This paper presents the synthesis and characterization of a novel monolayer transition metal dichalcogenide (TMD) called Janus SMoSe. The material is synthesized by controlled sulfurization of monolayer MoSe₂, where the top layer of selenium atoms is substituted by sulfur atoms while the bottom layer remains intact. The unique S-Mo-Se tri-layer atomic structure exhibits distinct Raman vibration peaks that can be distinguished from pure MoS₂, MoSe₂, and randomly alloyed MoSₓSe₂₋ₓ. The experimental Raman peaks agree well with predictions from density functional theory (DFT) simulations. Transmission electron microscopy and time-of-flight secondary ion mass spectrometry further confirm the tri-layer structure. DFT calculations predict the bandgap of Janus SMoSe, which is consistent with experimental photoluminescence spectroscopy results. The material exhibits high basal plane hydrogen evolution reaction (HER) activity, which is attributed to the synergistic effects of intrinsic defects and structural strain in the Janus structure. The HER efficiency of Janus SMoSe is higher than that of pure MoSe₂ and MoS₂, suggesting its potential as a promising catalyst for hydrogen evolution.