Tacrolimus (also FK-506 or fujimycin, trade names Prograf, Advagraf, Protopic) is an immunosuppressive drug used mainly after allogeneic organ transplant to lower the risk of organ rejection. It achieves this by inhibiting the production of interleukin-2, a molecule that promotes the development and proliferation of T cells, which are vital to the body's learned (or adaptive) immune response.

Tacrolimus is also used in the treatment of other T cell-mediated diseases such as eczema (for which it is applied to the skin in a medicated ointment), severe refractory uveitis after bone marrow transplants, exacerbations of minimal change disease, Kimura's disease, and the skin condition vitiligo.
Chemically it is a 23-membered macrolide lactone that was first discovered in 1987 from the fermentation broth of a Japanese soil sample that contained the bacterium Streptomyces tsukubaensis.