With nearly 98 percent of votes counted, Orban's Fidesz party has won 48.81 percent of the vote, the National Election Office (NVI) said, surpassing even the expectations of many within Fidesz.
In a tweet sent on Monday evening, Harper congratulated Orban for winning "a decisive fourth term" and added that he looked forward to working with the Hungarian prime minister in his role as chairman of the International Democratic Union.
Borissov, in his message to Orbán, said: "I highly appreciate the friendly relations between Bulgaria and Hungary and I am confident of the excellent prospects for the development of fruitful co-operation in bilateral, regional and multilateral terms".
In Hungary, the electorate's economic anxieties and general discontent with the political system, both before and after the 2010 election, have allowed Fidesz to implement these radical changes without provoking effective public opposition.
The proposed legislation is part of Orban's campaign targeting Hungarian born USA financier George Soros, whose philanthropy aims to bolster liberal and open-border values.
France' National Front head, Marine Le Pen, said Orban's victory proved that "the inversion of values and mass immigration advocated by the European Union are rejected again".
German Chancellor Angela Merkel congratulated Orban and called for cooperation, her spokesman Steffen Seibert told a news conference.
In Orban-era Hungary, everything revolves around the principle of the ethnically-determined nation: from family and media policy to foreign policy, it's Hungary and Hungarians first. "We would like to keep Hungary as Hungary", he said.
In the election campaign, Orban drove home the far right's xenophobic, anti-immigration pledges about protecting natives from an onslaught of risky and impure outsiders, especially Muslims.
The final vote count confirmed Mr Orban had won a 134-seat "supermajority" in Hungary's 199-seat assembly, opening the door to the so-called "Stop Soros" laws named after George Soros, the billionaire liberal philanthropist, who was targeted in the campaign.
The right-wing nationalist Jobbik party (the Movement for Better Hungary) is second with 20% and 26 deputies. In an interview with Spanish TV channel Intereconomia in 2015, Orban raised fears about integrating Muslim migrants in the European Union when he said: "What sort of Europe do we want to have?"
In an editorial in the weekly Heti Valasz before the vote, editor-in-chief Borokai cited Orban's recent track record in a decades-long list of political disappointments, saying the key question of the election was whether it would produce any change.
A staunch supporter of establishing close relations with Russian President Vladimir Putin and a popular figure with other nationalist populists, including US President Donald J. Trump and former Slovak President Robert Fico, Orban vowed to defend Hungary from Muslim migrants and renewed his call for keeping Muslims out of Europe.
The left opposition secured the majority of the capital Budapest, but Fidesz swiped the rest of the country.
"It's a confirmation of Central Europe's emancipation policy", Szymanski told the private TVN-24 broadcaster when asked what Orban's victory means for the EU. The government said the presence of the migrants would weaken Hungary's security and increase its risk of terrorism.