转化（英语：transformation，又译转型）即细胞通过摄取外源遗传物质(DNA或RNA)而发生遗传学改变的过程。在转化过程中，转化的DNA片段称为转化因子。受体菌只有处在感受态时才能够摄取转化因子。转化因子通常是质粒DNA。而质粒融合或病毒感染是导致引入外源DNA的原因。动物细胞的转化又被称为转染(transfection)。转基因植物的产生通常也被认为是一种转化。

在分子生物学实验中，细胞通过特殊处理后可变成感受态细胞(competent cells)，即可以接受外源DNA。有两种常用方法：

In molecular biology, transformation is the genetic alteration of a cell resulting from the direct uptake and incorporation of exogenous genetic material (exogenous DNA) from its surroundings through the cell membrane(s). Transformation occurs naturally in some species of bacteria, but it can also be effected by artificial means in other cells. For transformation to happen, bacteria must be in a state of competence, which might occur as a time-limited response to environmental conditions such as starvation and cell density.