Sirolimus (INN) is a relatively new immunosuppressant drug used to prevent rejection in organ transplantation, and is especially useful in kidney transplants. It is also known as rapamycin. Sirolimus is a macrolide antibiotic ("-mycin") first discovered as a product of the bacterium Streptomyces hygroscopicus in a soil sample from an island called Rapa Nui, better known as Easter Island. It is marketed under the trade name Rapamune by Wyeth.

Interestingly, sirolimus was originally developed as an antifungal agent. However, this was abandoned when it was discovered that it had potent immunosuppressive and antiproliferative properties.