© AFPMr Berlusconi said he had "nothing to be ashamed of"
The girl, named as Iris Berardi, was known to police as a prostitute, Italian reports say, and attended one of the events days before turning 18.
The details have emerged from new documents submitted by prosecutors on Wednesday to a parliamentary committee.
Mr Berlusconi says the allegations against him are politically motivated.
Using the services of prostitutes is not a crime in Italy, but paying a prostitute aged under 18 is an offence.
Ms Berardi's presence at Mr Berlusconi's villas in Sardinia and Milan has been proven by mobile phone records, Italian media report.
According to Ansa, Ms Berardi attended a party at Porto Rotondo, in Sardinia, on 21 November 2009 - just a few days before her 18th birthday. She also attended a party at his villa at Arcore, near Milan, in December the same year.
Ten days ago, prosecutors submitted a 389-page document, in which they said they had proof of payments made by the billionaire leader to prostitutes. The latest document runs to 227 pages.
The embattled 74-year-old has refused to attend questioning on the matter and has vowed to punish the magistrates who, he insists, are pursuing a political vendetta against him.
Much of the first dossier focuses on Karima El Mahroug, an 18-year-old Moroccan belly-dancer who attended Mr Berlusconi's parties when she was 17 and, prosecutors say, was paid to have sex with him.
She and Mr Berlusconi have denied having sexual relations and Ms Mahroug has described a sum of 7,000 euros (£6,000) from the prime minister as a "gift".
I think the old man is having the time of his life. I just noticed BBC is the source. Under Italian law being 17 a woman is considered an adult and sexually mature. Therefore she can be a prostitute or do anything else she wants. Although not legal usually the police look the other way in such matters. The trial will most probably be a mockery like so many others.