丝裂原（英语：Mitogen，又称促分裂原、促细胞分裂剂）是促进细胞开始分裂，触发有丝分裂的化学物质，通常是蛋白质形式。

A mitogen is a chemical substance that encourages a cell to commence cell division, triggering mitosis. A mitogen is usually some form of a protein. Mitogenesis is the induction (triggering) of mitosis, typically via a mitogen. Mitogens trigger signal transduction pathways in which mitogen-activated protein kinase (MAPK) is involved, leading to mitosis.