Nicola Zingaretti is now the new leader of Italy's centre-left Democratic Party.
The 53-year-old president of the Lazio region received more than 65 percent of the votes during the primary election over the weekend.
The party is the main opposition to the coalition government between the far-right League and anti-establishment Five Star Movement.
More than 1.7 million Italians voted according to officials, which is more than expected.
The group will now work towards re-establishing itself as a strong force against the populist government.
The election was triggered by the resignation of former Prime Minister Matteo Renzi when he stepped down after losing in the general election last year.
Now the challenge will be to unite the party and confront the far-right which is growing huge support.