More than 73% of voters backed Romano Prodi on 16 October as the candidate of the centre-left to face PM Silvio Berlusconi in next year's general elections.
Prodi called the primary election turn-out of more than four million voters an "incredible response, it's beyond every dream".
Six other contestants participated. The leader of the Rifondazione Comunista party, Fausto Bertinotti, came in second with 15% of the vote.
The vote followed the approval by the Italian parliament's lower house on 13 October of a controversial electoral reform, which, if backed by the senate, will reintroduce proportional representation.
It comes at a time when Berlusconi's ratings are low and is largely viewed as an attempt to strengthen his hand in next year's May elections.
