The Colombian Army says Jorge Briceno, one of the most senior leaders of the Revolutionary Armed Forces of Colombia (FARC), has been killed.
Briceno, also known as Mono Jojoy, died in combat in the country's eastern plains, Army said.
20 other rebels have been reportedly killed along Brineco.
"A military source has confirmed that it defeated a group of rebels during an operation involving the four branches of the military," the state-funded BBC quoted Colombia's attorney general, Guillermo Mendoza, as saying on Thursday.
Mendoza added that the Briceno's body has been found.
Briceno, who is believed to be the leader of FARC's military as well as drug trafficking operations, was responsible for a series of attacks on Colombian security forces throughout the 1990s, the Guardian reported.
Colombian President Juan Manuel Santos has called Jojoy's death "the hardest blow" in the history of the rebel group.