牛膝（学名：Achyranthes bidentata）为苋科牛膝属的植物。

多年生草本；圆柱形根；茎有棱角，节部膝状膨大；椭圆形至披针形的叶子对生；夏秋开绿色小花，穗状花序。

Achyranthes bidentata Blume (English common name: ox knee, Chinese: niu xi) is a species of flowering plant in the amaranth family, Amaranthaceae. It occurs in India, Nepal, China, and Japan. It is the source of huái niú xī, a Chinese herbal medicine with anti-inflammatory activities. In Nepal its root juice is used for toothache. Its seeds have been used as a substitute for cereal grains in famine years. The plant is used externally in the treatment of leech bites in Mizoram, India and a decoction as a diuretic.