With almost all votes counted, Berlusconi's conservative People of Freedom party (PDL) won 46.7% of the vote ahead of his center-left rival Walter Veltroni with 37.6% in the lower house, and by a similar margin in the upper house.
The 71-year-old who controls most of Italy's media through his company Mediaset said, "We have difficult months ahead that will require great strength," adding that his party needed to work with the opposition to introduce economic reforms.
Berlusconi has pledged to reduce the country's national debt, one of the world's largest, and cut taxes.
Walter Veltroni, a former Rome mayor, who succeeded ex-premier Romano Prodi after he resigned three months ago following the collapse of the parliamentary coalition, telephoned Berlusconi to congratulate him, but told journalists: "I don't know how long this majority will last."
Berlusconi's PDL is allied to controversial Umberto Bossi's Northern League party, which ran on an anti-immigration ticket and is expected to win around 8-9% of the vote. The league brought down Berlusconi's government in 1994 when the flamboyant leader was first elected prime minister.
An alliance of communists and environmental parties failed to win a single seat.