Lou Sullivan was an American author and activist known for his work for transgender men. In 1975, he transitioned, and left his hometown of Milwaukee for San Francisco. It is said he may have been one of the first trans men to publicly identify as gay.

He was a pioneer of the grassroots female to male movement, and had a huge part in supporting counseling and surgery resources outside of gender dysphoria clinics.

He founded FTM International, and the GLBT Historical Society. He also had a huge hand in lobbying for the recognition of gay trans men by the APA.