But the 78-year-old will not actually have to serve any time in prison because of legal technicalities, his lawyer, Niccolo Ghedini, said.
Ghedini slammed the verdict as "unjust and unjustified" but said the case would reach its statute of limitations on November 6th.
Since no legal action can be taken after that date, the guilty verdict will have no legal consequences for Berlusconi as there is not enough time for his case to come to be judged on appeal.
"It was a good trial, passionate, but in terms of consequences the imminent expiration date takes all the pathos out of the verdict," said prosecutor Henry John Woodcock.
The case revolved around an alleged plot to destabilise a 2006-08 centre-left government.
Bribes totalling €3 million were paid through an intermediary to Senator Sergio De Gregorio to get him to leave the coalition of then premier Romano Prodi.
The idea was to further weaken an already fragile coalition and the defection was widely seen as hurting Prodi's government, which collapsed in 2008, two years after it was elected.
Berlusconi's centre-right Forza Italia party won the resulting election and he served a third term as Italian premier until 2011.
The senator admitted accepting the money.
In summing up the bribery case, prosecutor Vincenzo Piscitelli had told the court the payoff was part of "a colossal economic investment made with the aim of achieving the sole goal that interested Berlusconi, who was obsessed by his desire to kick Prodi out and take his post".
The prosecution had called for a five-year sentence.
Berlusconi was not in court to hear the verdict, delivered by tribunal president Isabella Romani.
'Bunga Bunga' storm
The case was just one in a string of legal woes for the media tycoon.
Earlier this year, he completed a community service order for corporate tax fraud but was cleared of having sex with an under-age dancer known as Ruby the Heart-stealer after judges ruled he could not have known she was a minor.
Berlusconi would have gone to prison for the tax fraud had he been younger but was allowed to serve his time by helping out once a week with dementia sufferers at an old people's home.
In another headache for Berlusconi, prosecutors in Milan recently concluded a probe into evidence given by Ruby and other young women who attended sex parties at Berlusconi's lavish properties.
The inquiry determined that he paid Ruby and other young women some €10 million for their silence over what went on at his so-called Bunga Bunga parties.
Prosecutors said late last month that they had established he gave Ruby gifts and cash worth seven million euros as well as accommodation and the covering of medical bills.
Other witnesses received a further €3 million worth of enticements to keep quiet and in total 34 people, 21 of them young women, are suspected of varying degrees of corruption and giving false evidence.
Berlusconi is almost certain to face trial over the allegations. His defence says the gifts are proof of nothing more than their client's generosity.