Tens of thousands of South Sudanese have watched the raising of the new country’s flag at an independence ceremony in the capital, Juba.
Salva Kiir signed the constitution and took his oath of office in front of the jubilant crowds, becoming president of the world’s newest nation.
Sudan’s President Omar al-Bashir and UN chief Ban Ki-moon were among dignitaries watching the events.
Sudan earlier became the first state to officially recognise its new neighbour.
The world’s newest nation was born at midnight local South Sudanese time (2100 GMT), the climax of a process made possible by the 2005 peace deal that ended a long civil war.
The south’s independence follows decades of conflict with the north in which some 1.5 million people died.