Hispanics in the American Civil War fought on both the Union and Confederate sides of the conflict. Not all the Hispanics who fought in the American Civil War were "Hispanic Americans" — in other words citizens of the United States. Many of them were Spanish subjects or nationals from countries in the Caribbean, Central and South America. Some were born in what later became a U.S. territory and therefore did not have the right to U.S. citizenship. It is estimated that approximately 3,500 Hispanics, mostly Mexican-Americans, Puerto Ricans and Cubans living in the United States joined the war: 2,500 for the Confederacy and 1,000 for the Union. This number increased to 10,000 by the end of the war.

Brigadier General Diego Archuleta, was a member of the Mexican Congress. He joined the Mexican Army to fight against the United States in the Mexican–American War. Later, he was appointed an Indian Agent by President Abraham Lincoln, and joined the Union Army during the American Civil War. Archuleta became the first Hispanic to reach the military rank of Brigadier General.