His fiancee found his body at the bottom of a swimming pool, but there was no sign of foul play.
King was a victim of police brutality in 1991, but the officers involved were acquitted the following year.
The verdict triggered clashes between rioters and police which left more than 50 dead, thousands injured and thousands of properties destroyed.
Rialto Police Captain Randy Deanda said on Sunday King was found "unresponsive" at the bottom of his pool and was pronounced dead at 06:11 (13:11 GMT).
He had been stopped for speeding on a dark street on 3 March 1991. The four LA police officers who pulled him over hit him more than 50 times with their batons, kicked him and shot him with stun guns.
The BBC's Alastair Leithead in Los Angeles says the iconic images of his beating had a huge impact on an already tense Los Angeles.
Eventually, the whole chain of events had a profound impact on the way race was dealt with in the US, our correspondent says.
Those iconic images of an African American man lying unarmed on the floor being beaten by white police officers had a huge impact on what was an already tense Los Angeles, certainly within the African American community, and the way that the police reacted to that element of the community.
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