World leaders have been recognising South Sudan's nationhood as tens of thousands of people watched an independence and flag-raising ceremony in the capital, Juba.
The south's independence follows decades of conflict with the north in which some 1.5 million people died.
US President Barack Obama said in a statement he was "proud to declare that the United States formally recognises the Republic of South Sudan as a sovereign and independent state upon this day, July 9 2011".
"A proud flag flies over Juba and the map of the world has been redrawn," he added. "These symbols speak to the blood that has been spilled, the tears that have been shed, the ballots that have been cast, and the hopes that have been realised by so many millions of people."
Other world leaders to acknowledge the new nation included the UK, Russia, South Africa and China.
The BBC reports that the President of Sudan, Omar al-Bashir, attended the independence ceremony and, in his speech, was the first leader to officially recognise the new neighbour.
He told the gathering, including heads of state from around the world, that Sudan had fulfilled its promises.
He said many had believed Khartoum would never let the oil-rich south secede. Now, he said it was time for the US to keep its word and remove sanctions from Sudan.
The BBC says the sanctions cause considerable difficulty for the Sudanese economy which, with the south's independence, has also now lost most of its oil.
South Sudan is the 193rd nation to be recognised by the United Nations.
New nation needs help
The former director of aid agency World Vision says meeting the needs of millions of people returning to the world's newest country is a daunting prospect.
Seth Le Leu, a New Zealander, says about four million people were displaced from South Sudan during decades of civil war.
He says they have been returning since the 2005 peace accord, mostly to squalid conditions near urban areas.
Mr Le Leu says World Vision is teaching people to farm and fish, as well as building schools for children.
But he says the international community and aid agencies must continue to support those returning.