The BBC’s Will Ross says the party has begun in the capital Juba
South Sudan has become the world’s newest nation, the climax of a process made possible by the 2005 peace deal that ended a long and bloody civil war.
Sudan’s President Omar al-Bashir and UN Secretary General Ban Ki-moon are among international dignitaries attending celebrations in the capital, Juba.
Sudan earlier became the first state to officially recognise its new neighbour.