Mesterolone, sold under the brand name Proviron among others, is an androgen and anabolic steroid (AAS) medication which is used mainly in the treatment of low testosterone levels. It has also been used to treat male infertility, although this use is controversial. It is taken by mouth.

Side effects of mesterolone include symptoms of masculinization like acne, scalp hair loss, increased body hair growth, voice changes, and increased sexual desire. It has no risk of liver damage. The drug is a synthetic androgen and anabolic steroid and hence is an agonist of the androgen receptor (AR), the biological target of androgens like testosterone and dihydrotestosterone (DHT). It has weak androgenic effects and weak anabolic effects, which make it useful for producing masculinization. The drug has no estrogenic effects.

Mesterolone was first described by 1966 and introduced for medical use by 1967. In addition to its medical use, mesterolone has been used to improve physique and performance, although it is not commonly used for such purposes due to its weak anabolic effects.