Opposition politician Peter Marki-Zay received 57 percent of the vote in a referendum in which opposition parties voted for a common candidate.
The vote was the second round, and six parties are taking part in the attempt to remove Viktor Orbán and his ruling conservative party Fidesz from power in the parliamentary elections, which are to be held in April next year.
"We want a new, cleaner, honest Hungary, not just replace Orbán or his party," Marki-Zay told jubilant supporters in Budapest after Sunday night's primary election victory.
Acknowledges defeated
Marki-Zay, 49, is the mayor of Hódmezővásárhely;
he is considered to belong to the conservative part of the opposition and is a practicing Catholic.
Against him stood Klára Dobrev, who is a member of the European Parliament and belongs to the Democratic Coalition Party on the left wing of the opposition.
She received 43 percent of the vote.
"From now on, I support Peter Marki-Zay," said Dobrev when congratulating the mayor on the election victory.
She called on the opposition to rally in a united front, following a rather divisive election campaign during the primary election.
Been divided
The hitherto divided opposition in Hungary has not previously been able to agree to challenge Viktor Orbán, which has meant that Fidesz has been able to retain power with more than half of the seats in parliament.
The first primary round was held in September.
Marki-Zay then came in third place.
After the primary election, he succeeded in persuading the mayor of Budapest, Gergely Karácsony, to withdraw his candidacy.
Liberale Karácsony was previously seen as the favorite to lead the opposition in the parliamentary elections.