The UN General Assembly has admitted South Sudan as its 193rd member.
South Sudan is the first country to join the UN body since Montenegro in 2006, and the day was described by assembly president Joseph Deiss as a "historic and joyous moment".
"Welcome, welcome South Sudan to the community of nations," said UN Secretary General Ban Ki-moon.
The vote was unanimous and was immediately followed by applause in the General Assembly.
In a meeting on Wednesday, the United Nations Security Council voted unanimously to recommend South Sudan's membership of the world body.
South Sudan became independent on Saturday, after its people voted to secede from the north in January's referendum.
South Sudan says it is launching its own currency and the South Sudan pound note will be in circulation by next Monday.
However, Mr Ban warned the world's newest country that the hard work begins now.
Although it is oil-rich, it is one of the least developed areas of the world - only 16% of its women can read and write and there are very few paved roads in a country larger than Spain and Portugal combined.
Its independence follows decades of conflict with the north in which some 1.5 million people died.
The two countries have still to decide on issues such as drawing up the new border and how to divide Sudan's debts and oil wealth.