The Well of Loneliness is a lesbian novel by British author Radclyffe Hall that was first published in 1928 by Jonathan Cape. It follows the life of Stephen Gordon, an Englishwoman from an upper-class family whose "sexual inversion" (homosexuality) is apparent from an early age. She finds love with Mary Llewellyn, whom she meets while serving as an ambulance driver in World War I, but their happiness together is marred by social isolation and rejection, which Hall depicts as typically suffered by "inverts", with predictably debilitating effects. The novel portrays "inversion" as a natural, God-given state and makes an explicit plea: "Give us also the right to our existence".

Shortly after the book's publication, it had become the target of a campaign by James Douglas, editor of the Sunday Express. Douglas wrote that "I would rather give a healthy boy or a healthy girl a phial of prussic acid than this novel." A British court judged it obscene because it defended "unnatural practices between women"; In England, it would not be published again for another three decades, in 1959. In the United States, the book survived legal challenges in New York state and in Customs Court.

Publicity over The Well of Loneliness's legal battles increased the visibility of lesbians in British and American culture. For decades it was the best-known lesbian novel in English, and often the first source of information about lesbianism that young people could find. Some readers have valued it, while others have criticised it for Stephen's expressions of self-hatred, and viewed it as inspiring shame. Although critics differ as to the value of The Well of Loneliness as a work of literature, its treatment of sexuality and gender continues to inspire study and debate.