Partial official results show opposition leader Peter Magyar's party dominating the vote, in a bombshell election result with repercussions around Europe and beyond.
''Thank you, Hungary!" Magyar posted on X, as thousands of his supporters thronged the banks of the Danube in Budapest.
With 60 per cent of the vote counted, Magyar's Tisza party had more than 52 per cent support to 38 per cent for Orbán's governing Fidesz party.
That proportion will change as more votes are counted.
"I congratulated the victorious party," Orbán told supporters in Budapest.
"We are going to serve the Hungarian nation and our homeland from opposition as well."
It's a major blow for Orbán, the European Union's longest-serving leader and one of its biggest antagonists, who has travelled a long road from his early days as a liberal, anti-Soviet firebrand to the Russia-friendly nationalist admired today by the global far-right.
Turnout by 6:30pm was over 77 per cent, according to the National Election Office, a record number in any election in Hungary's post-Communist history.
"I'm asking our supporters and all Hungarians: Let's stay peaceful, cheerful, and if the results confirm our expectations, let's throw a big, Hungarian carnival," Magyar said.
Orbán has repeatedly frustrated EU efforts to support Ukraine in its war against Russia's full-scale invasion, while cultivating close ties to President Vladimir Putin and refusing to end Hungary's dependence on Russian fossil fuel imports.
Recent revelations have shown a top member of his government frequently shared the contents of EU discussions with Moscow, raising accusations that Hungary was acting on Russia's behalf within the bloc.
The election was closely watched in countries around Europe and beyond, which is a testament to the outsize role Orbán occupies in far-right populist politics worldwide.
Members of Trump's "Make America Great Again" movement are among those who see Orbán's government and his Fidesz political party as shining examples of conservative, anti-globalist politics in action.
After casting his vote, Magyar told reporters that the election was "a choice between east or west, propaganda or honest public discourse, corruption or clean public life".
Casting his ballot in Budapest, Marcell Mehringer, 21, said he was voting "primarily so that Hungary will finally be a so-called European country, and so that young people, and really everyone, will do their fundamental civic duty to unite this nation a bit and to break down these boundaries borne of hatred".
During his 16 years as prime minister, Orbán has launched harsh crackdowns on minority rights and media freedoms, subverted many of Hungary's institutions and been accused of siphoning large sums of money into the coffers of his allied business elite, an allegation he denies.
He also has heavily strained Hungary's relationship with the EU.
Although Hungary is one of the smaller EU countries, with a population of 9.5 million, Orbán has repeatedly used his veto to block decisions that require unanimity.
