The case centers on an exotic dancer nicknamed “Ruby the heart-stealer.”
A panel of judges sentenced him to seven years in prison. He is also barred him from holding public office.
Berlusconi can appeal.
Prosecutors had argued that Berlusconi had sex 13 times with underage dancer Karima el Mahroug and abused his position when he intervened in May 2010 to get her released from jail, where she was being held on charges of theft.
El Mahroug has said that she never had sex with Berlusconi and that she lied to him about her age, telling him she was 24.
The underage sex trial verdict is the latest in a string of legal troubles the former prime minister has faced.
Read more of this story here