环孢素（英语：Cyclosporine、Cyclosporine A、Ciclosporin）也称为“环孢菌素”或“环孢霉素”，是一种被广泛用于预防器官移植排斥的免疫抑制剂。它借由抑制T细胞的活性跟生长而达到抑制免疫系统的活性。环孢素于1969年由挪威Sandoz制药公司科学家于土壤样本中的真菌——多孔木霉（Tolypocladium inflatum）中首次分离出来。

虽然大部分的多肽都是由核糖体合成的，但环孢素这种具有11个胺基酸并非由核糖体合成，且其中含有一个在自然界非常少见的D-氨基酸。

Ciclosporin, also spelled cyclosporine, is an immunosuppressant drug widely used in organ transplantation to prevent rejection. It reduces the activity of the immune system by interfering with the activity and growth of T cells.

It was initially isolated from the fungus Tolypocladium inflatum found in a soil samples in 1969 from Norway, by Hans Peter Frey. Most peptides are synthesized by ribosomes, but ciclosporin is a cyclic nonribosomal peptide of 11 amino acids and contains a single D-amino acid, which is rarely encountered in nature.