The 71-year-old billionaire could become the first head of government in the world to testify in criminal proceedings over secret US transfers of "terrorism" suspects, known as renditions.
Judge Oscar Magi also ruled that Romano Prodi, Berlusconi's predecessor, could be called to give evidence.
'Torture'
Twenty-six Americans, nearly all of them believed to be CIA agents, are being tried in absentia on charges of kidnapping an Egyptian-born imam in 2003.
Prosecutors say a CIA-led team kidnapped Hassan Mustafa Osama Nasr off the streets of Milan and secretly flew him to Egypt.
There, Nasr says he was tortured under questioning and held for years without charge before being released in 2007.
Speaking in the Egyptian city of Alexandria on Tuesday, Nasr said: "I was tortured for 14 and a half months... I suffer now from heart and kidney problems. I have psychological problems."
Nasr faces an arrest warrant in Italy on suspicion of "terrorist" activity.
Bush ally
Berlusconi, who has just begun his third term, was prime minister when Nasr disappeared. Berlusconi has defended the Italian spy agency against accusations of wrongdoing.
A strong ally of George Bush, the US president, Berlusconi has denied knowledge of any kidnapping plan, but has also opposed the trial, arguing it could hurt Italy's reputation in the global intelligence community.
Meanwhile, Nicolo Pollari, Italy's former spy chief, wants Berlusconi and other past officials, including Prodi, to testify about classified documents he says prove he had nothing to do with rendition.
Titta Madia, his lawyer, has said Pollari believed that 88 such documents exist.
Madia said the court ruling means Berlusconi and Prodi would be obliged to give evidence if called.
The judge noted, however, that his ruling on evidence from Berlusconi and Prodi could still be rendered irrelevant by a parallel case in Italy's constitutional court.
That court is weighing whether state secrecy rules were violated by the prosecutors investigating the case, something Italy's government hopes will invalidate the renditions trial.