It marks a key moment 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.
Polls opened at 6am and were scheduled to close at 7pm.
Orbán and his top challenger, Péter Magyar, arrived at separate polling stations in Budapest at nearly the same time to cast their votes.
Speaking to reporters outside, Orbán, 62, said the campaign had been "a great national moment on our side" and thanked activists and supporters for their work.
"I'm here to win," he said.
Turnout after the first 11 hours of voting was over 74 per cent, according to the National Election Office, a record number in any election in Hungary's post-communist history.
With still two hours until polls closed, 140,000 more voters had cast their ballots than during the entirety of the 2022 elections.
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 being 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."
"I urge all Hungarian citizens to exercise their right to vote," he said.
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.
Most recently, he blocked a 90-billion euro ($A147 billion) EU loan to Ukraine, prompting his partners to accuse him of hijacking the critical aid.
Yet after winning four consecutive elections with a two-thirds majority for his party in parliament, signs have emerged that Orbán's absolute control over Hungary's politics may be reaching its end.
Magyar has rapidly risen to become Orbán's most serious challenger.
The 45-year-old leader of the centre-right Tisza party, which is leading in independent polls, campaigned on issues affecting ordinary voters, including Hungary's faltering public health care and transportation sectors and what he describes as rampant government corruption.
A former insider within Orbán's Fidesz, Magyar broke with the party in 2024 and quickly formed Tisza.
Since then, he has toured Hungary relentlessly, holding rallies in settlements big and small in a campaign blitz that recently had him visiting up to six towns daily.
