Nikolić won 50.21% of total votes to Tadić's 46.77%, preliminary results released by the Serbian Election Commission showed. Turnout was less than 50%.
The non-governmental Centre for Free Elections and Democracy, whose observers monitor every election in Serbia, also said that the final estimate "shows without a doubt" that Nikolić was the winner.
"Ballots were cast by 46.3% of the electorate, which is the lowest turnout since 2004," the centre's director, Marko Blagojević, also remarked.
Nikolić heads the Progressive Party which took the largest number of votes - 24% - in parliamentary election on 6 May, which coincided with the first-round vote for the presidency. His will have 73 seats in the 250-seat Parliament. The second force was Tadić’s “Choice for a Better Life” coalition with 22% of the vote, or 67 seats. The Socialist party of Ivica Dačić won 14.5% of the vote and 44 seats.
Nikolić, who lost two earlier contests against Tadić, has taken a pro-European stance since 2008, when his party decided to split from the nationalist Serbian Radical Party. But his European credentials remain to be proven. The historic leader of the Radicals, Vojislav Šešelj, is standing trial for war crimes at The Hague.
According to analysts, Nikolić’s election may complicate the setting up of a government, which was reportedly almost agreed between Tadić and Socialist leader Dačić.
“Serbia under Nikolić must create confidence in its will to move towards Europe and partnership in the region,” twitted Swedish Foreign Minister Carl Bildt, who is one of the most authoritative Western specialists on Serbia.