The Lost Cause of the Confederacy (or simply Lost Cause) is an American pseudohistorical negationist myth that claims the cause of the Confederate States during the American Civil War was just, heroic, and not centered on slavery. First enunciated in 1866, it has continued to influence racism, gender roles, and religious attitudes in the Southern United States to the present day.

Lost Cause proponents typically praise the traditional culture of honor and chivalry of the antebellum South. They argue that enslaved people were treated well and deny that their condition was the central cause of the war, contrary to statements made by Confederate leaders, such as in the Cornerstone Speech. Instead, they frame the war as a defense of states' rights, and as necessary to protect their agrarian economy against supposed Northern aggression. The Union victory is thus explained as the result of its greater size and industrial wealth, while the Confederate side is portrayed as having greater morality and military skill. Modern historians overwhelmingly disagree with these characterizations, noting that the central cause of the war was slavery.

There were two intense periods of Lost Cause activity: the first was around the turn of the 20th century, when efforts were made to preserve the memories of dying Confederate veterans, and the second was during the civil rights movement of the 1950s and 1960s, in reaction to growing public support for racial equality. Through actions such as building prominent Confederate monuments and writing history textbooks, Lost Cause organizations (including the United Daughters of the Confederacy and Sons of Confederate Veterans) sought to ensure Southern whites would know what they called the "true" narrative of the Civil War, and therefore continue to support white supremacist policies such as Jim Crow laws. In that regard, white supremacy is a central feature of the Lost Cause narrative.