The General Assembly admitted South Sudan on Thursday as the 193rd member of the United Nations, Reuters reports.
Israel, last Sunday, extended formal recognition to South Sudan ahead of the vote.
The general vote, by acclamation, followed South Sudan's independence ceremony in the capital Juba on Saturday, after its people voted in a January referendum to break away from Sudan -- a decision accepted by Khartoum.
The move seals the new African country's independence after decades of conflict.
Applause broke out in the General Assembly as South Sudan became the first country to join the world body since Montenegro in 2006.
"Welcome, South Sudan. Welcome to the community of nations," UN Secretary-General Ban Ki-moon said. Assembly President Joseph Deiss said it was a "historic and joyous moment."
South Sudan's vote for independence was held under the terms of a 2005 peace deal that ended a 20-year war between north and south Sudan.
Sudan became independent in 1956 but was long plagued by conflict between its mainly Muslim Arabic-speaking north and its black African south, where many are Christian or follow traditional beliefs.
The Security Council, which has to rule on all UN membership applications, had recommended on Wednesday that the General Assembly admit South Sudan.