The incident took place on Friday night in the town of Segrate on the outskirts of Milan, during a rally for Paolo Micheli, who is running for mayor in local elections Sunday.
Supporters of Micheli, who has the backing of the ruling centre-left Democratic Party, watched agape as Berlusconi swept into the square where the gathering was taking place, flanked by six bodyguards.
The 78-year-old former premier wandered around the square for a few minutes before asking someone the candidate's name.
On being told the answer he said “Young people I'm counting on you to vote Paolo” only to beat a hasty retreat after being informed of his mistake by a member of his entourage.
“With the support of the Cavaliere I can say I've won,” Micheli reacted in an ironic post on his Facebook page.
Berlusconi appeared to have gone astray while looking for the hall where the mayoral candidate of his Forza Italia party, Tecla Fraschini, was holding a campaign event.
Twenty million voters go to the polls on Sunday to elect governors in seven of Italy's 20 regions, as well as the mayors of more than 1,000 municipalities.
The election is seen as a test for Renzi and for Berlusconi's fading Forza Italia, which faces a growing challenge for right-wing votes from the anti-immigration Northern League.