结晶紫（crystal violet）或称龙胆紫（gentian violet），也称「甲基紫10B」，是一种三苯甲烷系染料。使用于组织学染色，也用在革兰氏染色试验中以区别不同类的细菌。具有抗菌、抗真菌、驱虫的性质，因此早年用作外用杀菌剂。

龙胆紫一词原指甲基对蔷薇苯胺染料（methyl pararosaniline dyes，即甲基紫）的混合物，但现今多视为结晶紫的同义词。龙胆紫的名称来自于颜色与龙胆花瓣相似，亦可由龙胆花提炼。

Crystal violet or gentian violet (also known as methyl violet 10B or hexamethyl pararosaniline chloride) is a triarylmethane dye used as a histological stain and in Gram's method of classifying bacteria. Crystal violet has antibacterial, antifungal, and anthelmintic properties and was formerly important as a topical antiseptic. The medical use of the dye has been largely superseded by more modern drugs, although it is still listed by the World Health Organization.