Sanmenxia (; postal: Sanmenhsia) is a prefecture-level city in western Henan Province, China. The westernmost prefecture-level city in Henan, Sanmenxia borders Luoyang to the east, Nanyang to the southeast, Shaanxi Province to the west and Shanxi Province to the north. The city lies on the south side of the Yellow River at the point where the river cuts through the Loess Plateau on its way to the North China Plain.