European Commission chief Ursula von der Leyen congratulated Viktor Orban's election opponent in a matter of minutes following his victory on Sunday night.
Mr Orban had served as the leader of Hungary for 16 years after his party Fidesz came into power - but was turfed out by former colleague Peter Magyar yesterday evening.
Shortly after Mr Orban conceded defeat, Mrs von der Leyen said: "Europe's heart is beating stronger in Hungary tonight."
She added on social media: "Hungary has chosen Europe. Europe has always chosen Hungary... A country reclaims its European path. The Union grows stronger."
Tensions have long simmered between Mr Orban and the EU chief over his blocking for an aid deal for Ukraine - and frequently clashed with its President Volodymyr Zelensky.
Mr Magyar, the leader of the Tisza party, promised to back the deal, and said Hungary wants to be a "European country again" on the campaign trail.
Mr Orban has labelled him a "puppet" of the EU and Ukraine in the past - while the new PM has long called for Hungary to adopt the Euro.
Just days ago, US Vice President JD Vance had warned the EU was committing "foreign election interference" on a visit to Hungary.
Viktor Orban has ruled Hungary since 2016
|REUTERS
Mr Vance said "outside forces" were exerting influence on the election, endorsing Mr Orban while saying European leaders "hate this guy".
He said: "We want you to make a decision about your future with no outside forces pressuring you or telling you what to do.
"I'm not telling you exactly who to vote for - but what I am telling you is that the bureaucrats in Brussels, those people should not be listened to."
Responding to the US Vice President's visit, Mr Magyar said: "No foreign country may interfere in Hungarian elections."
He added: "Hungarian history is not written in Washington, Moscow, or Brussels - it is written in Hungary’s streets and squares."
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Peter Magyar said in his victory speech that he planned to visit Brussels to 'bring home the EU funds that the Hungarian people are entitled to'
|REUTERS
Sir Keir Starmer also congratulated Mr Magyar on his victory.
He said: "This is an historic moment, not only for Hungary, but for European democracy.
"I look forward to working with you for the security and prosperity of both our countries."
While UK Government insiders told The Spectator that Britain was hoping Hungary "will now return to the European fold".
Hungary's Prime Minister-designate laid out his first international visits in his first speech.
US Vice President JD Vance warned of 'outside forces' interfering in the election as he endorsed Viktor Orban
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He said he first planned to visit Poland, then Austrian capital Vienna and a third to Brussels - to "bring home the EU funds that the Hungarian people are entitled to".
Hungary has been barred from receiving €17billion in EU cash by the bloc over concerns of "democratic backsliding" and judicial independence.
Mr Magyar also called for all "the puppets who have been in power for the past 16 years" to leave their office in his initial speech.
Sir Keir was not the only British party leader to congratulate Mr Magyar.
EU Commission President Ursula Von der Leyen was among the first to congratulate Peter Magyar for his electoral victory
|GETTY
Green Party leader Zack Polanski seized the chance to turn developments in Europe into a swipe at Reform UK, crowing: "Orban’s defeat shows that when we stand together against the politics of hate and division, hope wins and the far right friends of Farage can be defeated."
Liberal Democrat chief and outspoken Donald Trump critic Sir Ed Davey used the defeat to take a shot at Mr Vance.
He said: "Has anyone noticed that wherever JD Vance goes, he just makes a mess.
"In Munich he insulted European allies. In Greenland he turned everyone against Trump. And now he’s helped Viktor Orban lose re-election," Sir Ed said.