Berlusconi, 76, told reporters in Milan he was stepping into the breach because of a lack of effective leadership within his People of Freedom (PDL) party and was "running to win."
"It's not that we did not look ... but one needs time to be imposed as leader," Berlusconi said.
The BBC said Berlusconi was referring to the lack of popular traction for PDL Secretary General Angelino Alfano.
He also said he was running because he considered the austerity policies of Prime Minister Mario Monti to be harmful to Italy.
The latest campaign will likely cap a colorful and controversial political career for Berlusconi. He has served three terms as prime minister, building up what is believed to be a sizable personal fortune at the same time.
Berlusconi resigned from the job in 2011 after being accused of tax fraud. He has been at the center of a scandal involving an underage prostitute, although he has denied any wrongdoing.