蓖麻（学名：Ricinus communis；英语：castor oil plant）是一种大戟科植物，是蓖麻属（Ricinus）下的唯一物种。

蓖麻是一年或多年生草本植物。全株很光滑，上被蜡粉，通常呈绿色、青灰色或紫红色；茎圆形中空，有分枝；叶互生较大，掌状分裂；圆锥花序，单性花无花瓣，雌花着生在花序的上部，淡红色花柱，雄花在花序的下部，淡黄色；蒴果有刺或无刺；椭圆形种子，种皮硬，有光泽并有黑、白、棕色斑纹。

Ricinus communis, the castorbean or castor-oil-plant, is a species of flowering plant in the spurge family, Euphorbiaceae. It is the sole species in the monotypic genus, Ricinus, and subtribe, Ricininae. The evolution of castor and its relation to other species are currently being studied using modern genetic tools. It reproduces with a mixed pollination system which favor selfing by geitonogamy but at the same time can be an out-crosser by anemophily or entomophily.