Joseph Nicolosi (January 24, 1947 – March 8, 2017) was an American clinical psychologist who advocated and practised "reparative therapy", a form of the pseudoscientific treatment of conversion therapy that he claimed could help people overcome or mitigate their homosexual desires and replace them with heterosexual ones. Nicolosi was a founder and president of the National Association for Research and Therapy of Homosexuality (NARTH). Medical institutions warn that conversion therapy is ineffective and may be harmful, and that there is no evidence that sexual orientation can be changed by such treatments.

Nicolosi described his ideas in Reparative Therapy of Male Homosexuality: A New Clinical Approach (1991) and three other books. Nicolosi proposed that homosexuality is often the product of a condition he described as gender-identity deficit caused by an alienation from, and perceived rejection by, formative individuals of the subject's gender which interrupts normal masculine or feminine identification process. He also held that adaptation to gender trauma during formative years could alienate a child from their "fundamental nature." His goal was to restore "that which functions in accordance with its biological design.”