Povetkin, 37, improved his record to 32-1 (with 23 knockouts) with the victory in the 12-round bout on July 1 in Moscow, RFE/RL said.
Rudenko fell to 31-3 (19 knockouts).
The judges' scoring gave the Russian the victory by scores of 120-109, 120-108, and 120-108.
According to the Russian state-run TASS news agency, the bout was nearly stopped in the first round after Rudenko complained of a neck injury.
Read alsoUkraine bars top Russian boxer from championships over Crimea visit – mediaThe boxingscene.com website said the Ukrainian claimed he had been hit on the back of the neck, causing the injury.
He continued the bout after consultations with his doctors and the fight organizers.
Povetkin's victory comes a day after undefeated American Robert Easter defeated Russian Denis Shafikov, 32, in a unanimous decision in Toledo, Ohio, to maintain his International Boxing Federation lightweight title.