Moroccan-born exotic dancer El Mahroug, also known as "Ruby the Heartstealer", is said to have been paid for sex by the 76-year-old Berlusconi when she and dozens of other women attended his gatherings.
The age of consent in Italy is 14 but paying a woman for sex under the age of 18 is a crime. The dancer was 17 at the time of the alleged offences.
The judge at the trial in Milan ordered police to search for her in Italy and abroad. She is thought to have flown to the US last week and is not due back until after Christmas.
Her disappearance also prompted speculation as to who paid for her flights and accommodation.
Other witnesses that the defence had called, such as Hollywood actor George Clooney and footballer Cristiano Ronaldo, have also failed to show up.
Berlusconi went on trial last year over the sex allegations. Both he and El Mahroug deny any wrongdoing.
If convicted of having sex with an underage prostitute, Berlusconi faces a three-year jail sentence. He is also on trial for abuse of office.