Lieberman, who lost the Democratic primary to Greenwich businessman Ned Lamont, is running for re-election as an independent, a move that has alienated many Democrats.
Dan Gerstein, a spokesman for Lieberman's campaign, said the fundraiser will be held Nov. 1 and will be co-chaired another New York Republican, former Sen. Alfonse D'Amato, and former Democratic New York Mayor Ed Koch.
Bloomberg also will appear with Lieberman's wife, Hadassah, at an Oct. 25 fundraiser in Chicago and is hoping to do another appearance on Lieberman's behalf in Connecticut, Gerstein said.
"Senator Lieberman is extremely grateful to Mayor Bloomberg for being so generous with his time," Gerstein said. "The mayor is renowned as a leader who puts people ahead of politics and gets things done for his constituents and his support and validation mean a lot to our campaign."
Bloomberg spokesman Stu Loeser said, "The mayor thinks the world of Senator Lieberman, because he speaks his mind and does what he thinks is right."
Lamont spokeswoman Liz Dupont-Diehl said the fundraiser represents the kind of politics Lamont is trying to change.
"Ned Lamont is not accepting PAC money and is speaking out against the influence of lobbyist money," she said. "Joe Lieberman is attending fundraisers thrown by those very same lobbyists."