粉防己（学名：Stephania tetrandra），又名石蟾蜍，为防已科千金藤属的植物。含粉防己碱(Tetrandrine)。

落叶藤本。肥厚的块根呈圆柱状；宽三角状卵形叶片，盾状着生，截形或浅心形的基部，下面为灰白色；春末夏初开出很小的花，雌雄异株，雄花的头状聚伞花序，再排列成总状花序；球形核果。

Stephania tetrandra is a herbaceous perennial vine of the Menispermaceae family native to China and Taiwan. It grows from a short, woody caudex, climbing to a height of around three meters. The leaves are arranged spirally on the stem, and are peltate, i.e. with the leaf petiole attached near the centre of the leaf. Its root is used in traditional Chinese medicine (TCM).