This study provides the first comprehensive definition of the late Neogene to Quaternary deepwater fan system and gas hydrate (GH) petroleum system in the western South Korea Plateau (WSKP). An integrated geological and geophysical approach was employed to evaluate reservoir, source, and preservation potentials, establishing the WSKP as a promising target for GH exploration. The late Neogene to Quaternary deepwater fan system comprises submarine canyons, slope channel systems, and lobe complexes, with Lobe 1 and Lobe 2 identified as high-potential reservoirs. These lobe deposits demonstrated favorable reservoir properties, including maximum thicknesses of 15–38 m and 11–28 m, respectively, high lateral continuity, and elevated net-to-gross ratios. Source potential was attributed to two tectonostratigraphic units (MS2 and MS3), which exhibited high total organic carbon (>1.0 wt %) and hydrogen index values (>200 mgHC/gTOC) for bioigenic gas generation. Organic-rich claystones deposited under hemipelagic conditions served as effective source rocks. The biogenic gas window (20–60 °C) was observed at depths of 250–750 ms below the seafloor, corresponding to MS2 and MS3 intervals. Fault systems facilitated efficient gas migration, enabling upward transport from source rocks in MS2 and MS3 to reservoirs in MS4. Preservation potential was corroborated by seismic indicators such as bottom simulating reflectors, seismic attribute anomalies, and amplitude versus offset responses. This study offers a framework for understanding the GH petroleum system in the WSKP, advancing knowledge on depositional systems and GH accumulation. The findings contribute to understanding GH systems in back-arc basins and provide a foundation for future exploration.