With 99.9 per cent of votes counted, hard-right populist Calin Georgescu, 62, had 22.9 per cent, while centre-right contender Elena Lasconi was second with 19.16 per cent.
Lasconi pushed ahead of leftist Prime Minister Marcel Ciolacu, who was a clear favourite to win the first round, propelled by strong support from Romanian voters living abroad.
Romania's presidents have a semi-executive role that gives them control over defence spending - likely to be a difficult issue as Bucharest comes under pressure to uphold NATO spending goals during Donald Trump's second term as US president while trying to reduce a heavy fiscal deficit.
Some opinion polls had Georgescu running about five per cent of the vote in the run-up to the election after barely registering in earlier polls.
Elena Lasconi believes in raising defence spending and helping Ukraine. (EPA PHOTO)
Political commentator Radu Magdin said the difference between his single-digit popularity and Sunday's result was without precedent since Romania shed communism in 1989.
"Never in our 34 years of democracy have we seen such a surge compared to surveys," Magdin said.
Campaigning focused largely on the soaring cost of living, with Romania having the EU's biggest share of people at risk of poverty.
Ciolacu had courted voters with a promise of generous spending and no tax hikes, despite Romania running the European Union's largest budget deficit at eight per cent of economic outlook, while offering a sense of security in policy stability at a time of a war next door.
Formerly a prominent member of the hard-right Alliance for Uniting Romanians party, Georgescu has called NATO's ballistic missile defence shield in the Romanian town of Deveselu a "shame of diplomacy".
He has said the North Atlantic alliance will not protect any of its members should they be attacked by Russia.
"We are strong and brave, many of us voted, even more will do so in the second round," Georgescu said standing alone on Sunday evening.
Prime Minister Marcel Ciolacu had been a clear favourite to win the first round. (AP PHOTO)
Lasconi, a former journalist, joined the Save Romania Union in 2018 and became party head in 2024.Â
A two-term mayor, she believes in raising defence spending and helping Ukraine.
Romania shares a 650km border with Ukraine and since Russia attacked Kyiv in 2022, it has enabled the export of millions of tonnes of grain through its Black Sea port of Constanta and provided military aid, including the donation of a Patriot air defence battery.
Villages on the border with Ukraine have seen a barrage of drones breaching national airspace although no casualties have been reported.
One political commentator said Russian meddling to give Georgescu an edge could not be ruled out in the election.
"Based on Georgescu's stance towards Ukraine and the discrepancy between opinion surveys and the actual result, we cannot rule (that) out," said Sergiu Miscoiu, a political science professor at Babes-Bolyai University.
Outgoing two-term president Klaus Iohannis, 65, had cemented Romania's strong pro-Western stance but was accused of not doing enough to fight corruption.
Sunday's results are one of the biggest surprises in Romanian post-communist elections, with the leaders of the two largest parties, the leftist Social Democrats and centre-right Liberals - which are in a coalition government - eliminated in the first round.
