The judges said "criminal behavior" exhibited by Berlusconi led him to "systematically contaminate" the case in which he was also convicted of using his influence to hide Karima 'Ruby' El Mahroug's profession, the ANSA news agency reported.
The judges said Berlusconi also put "heavy pressure" on police to release the Moroccan dancer from custody to "protect himself." He told police the teen, who was detained on a theft claim, was the niece of former Egyptian leader Hosni Mubarak and that he was intervening to avoid a diplomatic incident.
In June, the court sentenced Berlusconi to seven years in prison for paying El Mahroug for sex and abusing his office to cover it up. Having sex with an underage prostitute is illegal in Italy, but prostitution is not.
The court said prosecutors "proved" Berlusconi paid El Mahroug for sex and that he knew she was a minor "beyond any reasonable doubt."
Berlusconi's lawyers called the court's explanation behind its conviction "surreal," ANSA said.
"It's in total contrast with the evidence, logic, and the fundamental principles of law, and the jurisprudence [afforded by] the supreme Cassation Court," defense attorneys Piero Longo and Niccolo Ghedini said in a statement.
Berlusconi also was banned for life from holding public office. The Italian Senate is set to vote next week to eject Berlusconi for his conviction in an unrelated case for tax fraud.