Cấm Sơn Reservoir () is a lake in Lục Ngạn District, Bắc Giang Province, Vietnam, near the border with Lạng Sơn Province. The lake lies off Highway 279, northeast of the city of Bắc Giang. It is one of the largest lakes in the Northern Vietnam region.
The lake has an area of some , but during the rainy season when flooding is common, the lake can expand to some . The length of the lake is about . Its widest point is about and narrowest point is about .
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