The African nation, which gained independence Saturday, became the U.N.'s 193rd member by acclamation Thursday.
General Assembly President Joseph Deiss banged a gavel signifying South Sudan's admission to the world body as diplomats burst into applause.
The country's independence was the climax of a 2005 peace agreement that ended decades of civil war with the Arab-dominated north and called for a referendum in which South Sudan voted overwhelmingly for secession.
After the vote, the flag of South Sudan was raised outside U.N. headquarters to more applause.
