Jorge Briceno, also known as Mono Jojoy, was considered the top military commander of the Farc, or Revolutionary Armed Forces of Colombia.
He was believed to be responsible for their military development and the brains behind many of the victories of the Rebel army.
In 2008 he was considered to be in frame to lead Farc after their founder and legendary leader, Manuel "Sureshot" Marulanda, died of a heart attack aged 78.
The position went instead to Alfonso Cano, long the movement's ideologue.
Jojoy remained in command of the Farc's strongest fighting unit, the Eastern Bloc.
For almost 10 years the Colombian army has had a task force dedicated to hunting the man known as the Farc field marshal, but on a number of occasions he slipped through their nets.
His death will be a major blow to the rebel group and a coup for Colombian President Juan Manuel Santos, says the BBC's Jeremy McDermott in Medellin.
The Marxist Farc rebels have been fighting the Colombian authorities since the mid-1960s.
BBC News
