Carlos Castaño Gil (15 May 1965 – 16 April 2004) was a Colombian paramilitary leader who was a founder of the Peasant Self-Defenders of Córdoba and Urabá (ACCU), a far-right paramilitary organisation in Colombia and a former member of the Medellin Cartel. Castaño and his brothers Fidel and Vicente founded the ACCU (and its previous incarnations) after their father was kidnapped and killed by the Revolutionary Armed Forces of Colombia (FARC), in association with other enemies or victims of the guerrillas. The ACCU later became one of the founding members of the United Self-Defense Forces of Colombia (AUC).

Carlos Castaño Gil was the youngest son of the family Castaño Gil, a rich landowner who would have been close to the Alianza Americana Anticomunista. Like his brothers, he experienced firsthand the murder of his father. Led by FARC Guerrilla, at 16 years old, he was determined to take up arms against FARC In revenge of his father's murder. He became a member of a self-defense group, which were among the first of their kind.

He was introduced to the Medellin Cartel kingpin Pablo Escobar by his brother Fidel, but he was against drugs. Castaño received combat training from his brother, from army officers and members of the paramilitary group as well as from the Israeli mercenary Yair Klein. Of this training came out the order for the extermination against the UP between the 1985-96

The Castaño's paramilitary groups were financed by drug kingpin José Gonzalo Rodriguez Gacha. The friendship between the Castaño brothers and Pablo Escobar broke after the murders of Galeano and Moncada clan (also Castaño's partners) while being detained inside his personal prison "La Catedral". Castaño and his brothers then became founders of the group "Los Pepes" (an acronym for "people persecuted by Pablo Escobar"). During this time Carlos was known by the aliases of "the Phantom" or "the Kid". Contacts between Los Pepes and the Colombian National Police Search Bloc allegedly resulted in Escobar's death.

One month after Escobar's death, Fidel Castaño was killed in a battle against EPL guerrillas. However, it is believed that Carlos may have had a role in Fidel's death in retaliation for the alleged rape and murder (or possible suicide) of a woman alleged to have been a lover of both brothers. After Fidel's death, Carlos Castaño assumed leadership of AUC, a paramilitary federation.

In a 1996 interview with writer Robin Kirk, later published in More Terrible Than Death: Massacres, Drugs and America's War in Colombia (PublicAffairs: New York, 2003), Castaño acknowledged that the men under his command committed "excesses", but defended them as necessary in Colombia's conflict. "Look, the guerrillas hide themselves within the civilian population, they manipulate the population". In a September 1997 interview in El Tiempo newspaper, Castaño admitted responsibility for the Mapiripán massacre.

In 1997, Castaño later founded an umbrella organization of paramilitaries operating in Colombia known as the United Self-Defense Forces of Colombia (AUC). The AUC demobilised in 2006 admitting to several brutal murders to the Colombian population. The AUC was accused by human rights organisations of committing atrocities, and it has openly admitted to its involvement in the drug trade. The AUC was listed by the US Department of State as a Foreign Terrorist Organisation. AUC was disbanded after then Colombian President Alvaro Uribe Vélez reached a peace agreement with reduced terms for its members.

Castaño was convicted in absentia of the murder of journalist Jaime Garzón, and sentenced to 38 years in prison.