Middleweight king Canelo, 29, had stepped up to face the 36-year-old Russian with two weight divisions. It proved to be a close fight that failed to ignite before the night of Kovalev ended with an explosive combination from Canelo. In four weight divisions, Canelo has now won world titles.
A cage fight at the MGM Grand seemed to be headed for a 12th round and one for the judges to determine before a left-hand hook-right combination came from nowhere and left Kovalev floundering on the ropes.
At that point, the judges had Canelo on their scorecards slightly ahead.
In his 16th consecutive world title fight, Kovalev was a tough opponent, a streak that started when he defeated Briton Nathan Cleverly in Cardiff in 2013.
But the victory means that in 56 fights Canelo still has just one loss-in 2013 against Floyd Mayweather.