Nicusor Dan gets 54 percent of the votes, while Simion gets 46 percent, show the preliminary figures when almost all votes are counted.
Dan says on Sunday evening that "part of society has won, a part that wants a comprehensive change in Romania".
There is also a part of society that lost today's election. A group that is rightly upset about how politics has been conducted in Romania, he says.
George Simion acknowledges defeat.
I want to congratulate my opponent, Nicusor Dan. He has won the election and this was the will of the Romanian people, says Simion in the early night.
The voter turnout was around 64 percent, compared to 53 percent in the first round, according to statistics from the Election Authority.
Romania's government accuses Russia of having attempted to influence Sunday's presidential election.
"During Romania's ongoing presidential election, we see again signs of Russian interference", writes a spokesperson for Romania's Foreign Ministry on X.
"A viral campaign with false news on Telegram and other social media platforms aims to influence the electoral process", continues the spokesperson.
Romania actually held its presidential election last autumn – but the country's political landscape was turned upside down. Another far-right candidate, Calin Georgescu, got the most votes in the first round – but the election was declared invalid due to allegations of Russian interference, including via TikTok.
Georgescu has not been allowed to stand in the new election, whose first round was held on 4 May.
