Andre Ward was knocked down early in the fight against Kovalev, but he came back to win on points and won the IBF, WBA Super and WBO world light-heavyweight titles.
The American boxer is still unbeaten after an unanimous points decision by the judges, winning 114-113 114-114 114-133 in Las Vegas.
Kovalev put Andre Ward on the mat with jab in the second round, with the loud crowd expecting him to win the encounter.
The Russian called for an immediate rematch and insisted politics played a hand in the first loss of his stella light-heavyweight career.
“It’s the wrong decision,” Kovalev told HBO in the ring following the outcome. “Witnesses are here. Everybody saw what happened.”
Ward said he would grant Kovalev a rematch, and Kovalev said he would accept one. Kovalev has the right to a rematch in his contract.