Naltrexone, a non-selective antagonist of opioid receptors, is mainly used as rehabilitation therapy for discharged opiate addicts to eliminate addiction in order to maintain a normal life and prevent or reduce relapse. In recent years, there have been some novel and significant findings on the off-label usage of naltrexone. Within a specific dosage window, LDN can act as an immunomodulator in multiple autoimmune diseases and malignant tumors as well as alleviate the symptoms of some mental disorders. The results of increasing studies indicate that LDN exerts its immunoregulatory activity by binding to opioid receptors in or on immune cells and tumor cells. These new discoveries indicate that LDN may become a promising immunomodulatory agent in the therapy for cancer and many immune-related diseases. In this article, we review the pharmacological functions and mechanisms of LDN as well as its clinical therapeutic potential as revealed by our team and other researchers.