Tadic competed with nationalist challenger Tomislav Nikolic in Sunday's run-off, and with 99.8% of ballots counted, had garnered 50.57% against Nikolic's 47.71%, according to the Electoral Commission. The remaining votes were declared invalid.
After the partial count was announced, Tadic told a crowd of supporters in Belgrade: "We want to go to Europe. We want to cooperate with the world."
The European Union's presidency holder, Slovenia, released a statement welcoming Tadic's re-election, and saying his endorsement would "accelerate its progress toward the EU, including candidate status."
The vote has come at a tense time for the Balkan nation, with its breakaway province of Kosovo set to unilaterally declare its independence within weeks.
In his campaign, challenger Nikolic had offered a course away from the EU, and toward closer ties with long-time ally Russia, which unlike Western powers has backed Belgrade's position on Kosovo.
The close result has been received by analysts as a sign of deep divisions in Serbia, between pro-Western elements looking to a future in the EU, and hardline nationalists.
The re-elected Serb leader told supporters: "We want to say to the people of Kosovo that we'll never let them down. We need to work together to fulfill Serbia's potential."
