Yuen Long Plain, Agricultural plain in New Territories, Hong Kong.

Yuen Long Plain is Hong Kong's largest alluvial plain, covering 144.3 square kilometres in the northwest New Territories. A ridge of hills near Au Tau divides it into a Yuen Long part (Yuen Long Town, Wang Chau, Shap Pat Heung, Tin Shui Wai, Lau Fau Shan, Ping Shan, Hung Shui Kiu, Ha Tsuen, Pak Nai) and a Kam Tin part (San Tin, Lok Ma Chau, Mai Po, Kam Tin, Shek Kong, Pat Heung).

Yuen Long Plain is Hong Kong's largest alluvial plain, formed between the Tang (618–907) and Song (960–1279) dynasties by river and tidal deposits. A 1979 geological map by the Public Works Department's Survey and Mapping Office shows it rests on Quaternary sediments over Mesozoic metamorphic and volcanic rock. Marble quarried here was once well known in Hong Kong.

The plain's name comes from the Cantonese 'Yuen Long', meaning 'deep and wide', a reference to the broad, fertile river floodplain on which it sits. Its villages, such as Ping Shan, Kam Tin and Shap Pat Heung, grew up on this sediment. Historically, large numbers of duck and pig farms operated across Yuen Long, and the area was long known for Yuen Long silk rice.

The plain is split into two parts by the hills near Au Tau: the Yuen Long section holds Yuen Long town centre, Wang Chau, Shap Pat Heung, Nam Sang Wai, Tin Shui Wai, Lau Fau Shan, Ping Shan, Hung Shui Kiu, Ha Tsuen and Pak Nai; the Kam Tin section holds San Tin, Lok Ma Chau, Mai Po, Kam Tin, Shek Kong and Pat Heung. Plan journeys around this divide, since the two halves are reached by different roads.

Yuen Long Plain was long known for duck and pig farms, and its marble was once famous in Hong Kong. The 1979 geological map by the Public Works Department's Lands and Survey Office showed the plain built on Quaternary Pleistocene to recent surface deposits, with metamorphic sedimentary and volcanic rocks of the Middle and Lower Jurassic exposed at Wang Chau and along the ridges from Tuen Mun to Ha Tsuen.