There was stunned silence when Berlusconi announced that he would vote with Letta.
"We have decided to vote for confidence, not without internal disputes," Berlusconi said. "This is not a U-turn".
He held his head in his hands after he spoke.
There were some guffaws, but mostly stunned silence in the chamber as Berlusconi made his climb-down, with Letta shaking his head.
Just 24 hours earlier, the ex-Premier had said that it would be "unthinkable" to back the Letta Government. But dozens of senators from the mogul's centre-right grouping said they would form a break-away centre-right party and support the left-right coalition Government as it sought to introduce electoral reforms and measures against Italy's spiralling unemployment.
The Pdl senator Robert Formigoni announced ahead of the vote that he and around 35 of his centre-right colleagues would back Letta in the confidence vote in defiance of Berlusconi.
"It's not an ideal Government," he said. "But it's better that than a devastating crisis that hits businesses and Italian families."
Letta, who had been tipped to win with just a handful of votes just minutes before Berlusconi's U-turn, ended up sweeping the vote with a crushing majority of 235 senators in favour and 70 against.
The new lease of life for Letta's coalition calmed the markets and increased Italy's chances of emerging from the economic doldrums, pundits said. Whether the Government will achieve the deep reforms and painful tax and budget measures needed to reverse a decade of economic stagnation and cut Italy's two trillion euro debt remains to be seen.
The fate of Berlusconi, who was forced to step down in November 2011 at the height of Italy's financial crisis, is gripping the country.
Commentators suggested his time as major political force was at an end.
Francesco Specchia, from the right-wing Libero newspaper, said: "In calling this confidence vote, for the first time ever, Berlusconi got his strategy completely wrong."
Giacomo Marramao, a politics professor at Roma Tre University, said: "I think we are seeing the final chapter of Berlusconi's political life."
Laura Puppato, a senator in Letta's centre-left Democratic Party, said: "We hope we are coming to the end of the era where government was run for the business and personal interests of one person."
Berlusconi, 77, is due to begin a one-year sentence of either house arrest or community service this month following his tax fraud conviction.Independent