In 2020, approximately 191,930 people were diagnosed with prostate cancer, and an estimated 33,330 people died from the disease. The most common therapies for prostate cancer involve androgen depletion via castration and/or drug treatment. However, the relapse and mortality rates of prostate cancer remain high, prompting a need for less invasive treatments. Now, a growing number of studies suggest that prostate cancer can be inhibited by the hormone melatonin. Melatonin, which is normally associated with the regulation of sleep, can inhibit prostate cancer partly by keeping androgen receptors out of cell nuclei, thus preventing excess androgen signaling and by activating MT1 signaling, ultimately reducing levels of the prostate cancer marker prostate-specific antigen. Melatonin also reduces tumor blood vessel growth and it regulates metabolism, affects differentiation, and induces death in prostate cancer cells. In patients and laboratory models, melatonin makes existing treatment methods more effective, and tracking urinary levels of melatonin may be useful for prostate cancer prediction. While more clinical studies are needed to clarify how the hormone inhibits prostate cancer, melatonin is a promising marker and treatment for this devastating disease.