The conservative and non-party outsider Peter Marki-Zay is expected to challenge the right-wing national prime minister Viktor Orbán as an opposition candidate in the parliamentary elections in 2022.
In a primary organized by the opposition, the 49-year-old was clearly ahead according to partial counting results from Sunday evening.
This is the first time such a code has taken place in Hungary.
Six previously divided opposition parties - from the left-green to the right-wing conservative - are supposed to support the winner in the election campaign against Orbán.
After counting more than half of the votes, Marki-Zay got 59.1 percent of the vote and his opponent Klara Dobrev from the social democratic party DK to 40.9 percent, as the pre-election commission announced on Sunday evening.
Dobrev already congratulated and promised to support him with full force in the parliamentary election campaign.
Marki-Zay studied economics, electrical engineering and history.
From 2004 to 2009 he lived with his family in Canada and the USA.
He only got into politics in 2018.
At that time he won - also surprisingly - the mayoral election in Hodmezövasarhely.
Until then, the place had been considered an impregnable stronghold of the Orbán party Fidesz.
Participation in the area code reached a record high: 662,016 voters voted within six days until Saturday evening.
633,811 citizens took part in the first round of the primary election at the end of September.
This value already exceeded the organizers' expectations.
There were five top candidates to choose from at the time.
In the first round, citizens also voted in 94 out of 106 constituencies on the respective common direct candidates for parliament.