Official results from nearly all voting stations show Dan garnering about 54 per cent of ballots cast by voters in the EU and NATO member country of about 19 million people, while Trump supporter George Simion was at 46 per cent.
Dan, 55, a soft-spoken mathematician, made a last-minute dash to the top in recent days after weeks of trailing Simion, a eurosceptic wanting to end military aid for Ukraine in its war with Russia.Â
The election drew the highest percentage of voter turnout in a Romanian election in 25 years.
Simion, who was the top vote-getter in the first round of the election two weeks ago with 41 per cent of ballots cast, conceded after earlier saying he won the election.
Dan had campaigned on a pledge to fight rampant corruption, to maintain support for Ukraine - where Romania has played an important logistic role - and to keep the country firmly within the European mainstream.
On Sunday evening local time in the capital Bucharest, Dan supporters chanted "Russia, don't forget, Romania isn't yours".
Staunchly pro-EU and NATO, Dan said in the run-up to the election that Romania's support for Ukraine was crucial for its own security against a growing Russian threat.
Ukraine's President Volodymyr Zelenskiy congratulated Dan on his "historic victory".
"For Ukraine - as a neighbour and friend - it is important to have Romania as a reliable partner," Zelenskiy wrote on social media.
The election showed backlash against politicians taking inspiration from right-wing populist Trump's Make America Great Again (MAGA) movement, according to Mujtaba Rahman, managing director for Europe at consultancy Eurasia Group.
"This is a really strong result for the pro-European candidate," Rahman said.
"It's another example of the positive Trump effect on European election cycles where concerns about the political and policy direction moving in a MAGA-like way have mobilised voters."
Speaking shortly after voting ended, Simion said his election was "clear".
"I won!!! I am the new President of Romania and I am giving back the power to the Romanians!" Simion said on Facebook. But Simion conceded the election later in the evening.
