Testosterone undecanoate is an androgen and anabolic steroid (AAS) medication which is used mainly in the treatment of low testosterone levels in men. Which includes hormone therapy for transgender men. It is taken by mouth two to three times per day with food or given by injection into muscle once every 3.5 days to 2 weeks, depending on individual response.

Testosterone undecanoate was introduced in China for use by injection and in Europe for use by mouth in the 1970s. It became available for use by injection in Europe in the early to mid 2000s and in the United States in 2014. A formulation for use by mouth is not currently available in the United States but is pending approval as of 2018. Along with testosterone enanthate, testosterone cypionate, and testosterone propionate, testosterone undecanoate is one of the most widely used testosterone esters. However, it has advantages over other testosterone esters in that it can be taken by mouth and in that it has a far longer duration when given by injection. In addition to its medical use, testosterone undecanoate is used to improve physique and performance. The drug is a controlled substance in many countries and so non-medical use is generally illicit.