It was the most damaging setback yet for the 76-year-old Berlusconi, who has been tried numerous times for his business dealings but never before for his personal conduct.
Still, he vowed that his days as a political force are not over. He has two levels of appeal and his supporters quickly rallied around him.
The charges against the billionaire media mogul resulted from what became widely known in Italy as "bunga bunga" parties hosted in 2010 by Berlusconi, then the sitting premier, at his villa near Milan, where he wined and dined beautiful young women.
Berlusconi's defense described the dinner parties as elegant soirees; prosecutors said they were sex-fueled gatherings that women were paid to attend. The woman at the center of the scandal, Karima el-Mahroug, better known as Ruby, has described aspiring showgirls stripping provocatively for the then-Italian leader.
Both Berlusconi and el-Mahroug denied ever having sex, and el-Mahroug says she never worked as a prostitute.
After the verdict, Berlusconi said in a message posted on Facebook that he believed he would be acquitted "because in the facts there is really no possibility to convict me."
He called the sentence "incredible, of a violence never seen or heard before, to try to eliminate me from the political life of this country." He pledged to "resist this persecution, because I am absolutely innocent, and I don't want in any way to abandon my battle to make Italy a truly free and just country."
The Milan criminal court's ruling was unexpectedly stiff, going further than the original charges and openly questioning whether many of the young women who testified in Berlusconi's defense had lied on the stand to protect him.
The panel of three judges, all women, said Berlusconi went beyond using his influence to cover up his relationship with the then-17-year-old Moroccan, as originally charged. They said he stepped in to win her release from police custody when she was accused of theft.
As a result, they added one year to the six requested by prosecutors.
The court also said it was turning over to prosecutors files containing the testimony of more than 30 young women who attended the now-infamous "bunga bunga" parties to investigate if they had lied under oath when they denied a sexual character to the gatherings.
Justice Minister Angelino Alfano, who is also secretary of Berlusconi's People of Liberty Party, said he told his political mentor to "hang in there, and keep moving on" in a phone call after the verdict.
Berlusconi was not in court for the sentencing, but his lawyer, Niccolo Ghedini, said he would appeal a decision he called both "largely expected" and "out of reality." The Berlusconi camp has long accused Milan magistrates of mounting a campaign to sideline him politically.
Berlusconi loyalist Daniela Santanche, who attended the sentencing, denounced it as "an outrage, and a political sentence that has nothing to do with justice." But she also said that it should have no impact on the government.
Some political opponents, however, said Berlusconi, who has shaped political discourse in Italy for two decades, should withdraw from politics immediately.
Alessandro Di Battista, a lawmaker in the anti-establishment 5 Star Movement, said Berlusconi "must go to jail. It is outrageous that he is a senator that can make laws. Until he goes to jail, the country is not free." And the left-leaning governor of Apulia, Nicchi Vendola, said Berlusconi should "abandon public life."
Berlusconi does not have any official role in government, but he is a senator in Parliament and retains influence in the uneasy grand coalition between his forces and the center-left Democratic Party that emerged after inconclusive February elections. The Democratic Party issued a statement acknowledging the sentence and in support of the autonomy of the courts.
Berlusconi's high-stakes judicial woes are far from over. He faces a final appeal in a tax fraud conviction for which he has been sentenced to four years in jail and a five-year ban from office.
Roberto D'Alimonte, a political analyst for il Sole 24 Ore daily and professor at Rome's LUISS University, said the tax fraud conviction poses the more immediate threat since Italy's highest court is likely to rule before the statute of limitations runs out.
The sex-for-hire case "weakens him politically, but not that much, because we have seen that his voter base seems to be insulated from the impact of these sentences. We saw in the last elections, everyone thought he was dead, but he came back to life," D'Alimonte said.
While the verdict drew intense international media coverage, with half a dozen satellite vans parked outside the Milan courthouse, there was only a smattering of public interest. A few anti-Berlusconi protesters gathered outside, and just a handful of citizens joined journalists crammed inside the small courtroom.
"For 20 years, he's been running Italy. He's done what he wanted," said Aurelio De Boni, a retired suit salesman from Milan who attended the trial.
Neither Berlusconi nor el-Mahroug testified in this trial. El-Mahroug was called by the defense but failed to show, delaying the trial, and Berlusconi's team eventually dropped her from the witness list.
El-Mahroug, however, did testify in the separate trial of three Berlusconi aides charged with procuring prostitutes for the parties. She told that court that Berlusconi's disco featured aspiring showgirls dressed as sexy nuns and nurses performing striptease acts, and that one woman even dressed up as President Barack Obama.
El-Mahroug, now 20, said in the other trial that she attended about a half-dozen parties at Berlusconi's villa, and that after each, Berlusconi handed her an envelope with up to 3,000 euros ($3,900). She said she later received 30,000 euros cash from the then-premier, paid through an intermediary money that she told Berlusconi she wanted to use to open a beauty salon, despite having no formal training.
But she denied ever receiving millions from the billionaire, as she had claimed to acquaintances, saying they were "lies" meant to inflate her own importance.
She was 17 at the time of the alleged encounters but passed herself off as 24. She also claimed she was related to then-Egyptian President Hosni Mubarak. Berlusconi's lawyers argued that he believed el-Mahroug was indeed Mubarak's niece, and he called police after she was accused of theft in a bid to avoid a diplomatic incident.
Berlusconi, who has been tried numerous times relating to his business dealings, has been convicted in other cases at the trial level. But those convictions have always either been overturned on appeal or else the statute of limitations has run out before Italy's high court could have its say.
The sex-for-hire case is the first involving his personal conduct.
Later this week, Italy's highest court has scheduled a hearing on Berlusconi's appeal to a verdict ordering him to pay 560 million euros ($800 million) to a rival media group over corruption in the acquisition of the Mondadori publishing empire. And a preliminary hearing will begin in Naples to decide if Berlusconi should be tried for allegedly bribing a lawmaker to bolt a previous center-left government under Romano Prodi and join his party, a move that weakened Prodi's slim majority.