Despite initial resistence from the government, the regional protection force will be deployed to the world's newest nation.
Sexual abuse, death, displacement and widespread violence have kept South Sudan teetering on the brink of civil war.
The announcement came from the South Sudan capital Juba after UN Security Council ambassadors met with President Salva Kiir in a rare visit to the turbulent African country.
About 12,000 UN peacekeepers are already in the country, but that will now be increased by 4,000.
South Sudan also committed to implementing a hybrid court to investigate war crimes.
The Security Council had authorised the deployment last month following an escalation of violence but could only enter South Sudan with the government's authorisation.
That bloodshed was caused by heavy fighting between troops loyal to President Kiir and forces backing former vice president Riek Machar.
The United States ambassador to the United Nations, Samantha Power, says other humanitarian issues made the additional numbers essential.
"We met with women who described a huge surge in sexual violence against women who leave the camp in order to try to get firewood, in order to be able to cook the food for their families for their children. As a mother, I can't imagine that choice, a choice in whether I cook for my kids or whether I risk sexual violence outside the camp. I know I would go and take that risk for my children. I think any mother would."
UN officials say the new force will need more than two months to deploy.
President Kiir's transitional government had been hesistant to accept the peacekeepers, claiming a breach of national sovereignty.
But the President was left with no choice other than yielding to the United Nations' will, due to the threat of an arms embargo.
Kiir loyalists argued a greater UN presence would affect what they called the "serenity" of the country.
But France's Deputy Permanent Representative to the UN, Alexis Lamek, told UN Radio the efforts are purely focused on the safety and welfare of the South Sudanese.
"It's not about serenity. It's about the people of South Sudan and the fact that the peace agreement (needs to)be finally implemented. It was signed one year ago. I think this starts by the South Sudanese, who really want to see this agreement implemented."
Nearly 200,000 people are living in UN protection sites, and hundreds of thousands more have fled South Sudan, all a result of more than two years of fighting.
Political rivalry between Mr Kiir and Mr Machar sparked the 2013 civil war, which has killed tens of thousands, with both government and rebel forces accused of widespread abuse.
The President and his former ally signed a shaky peace deal a year ago, but fighting has continued, and Mr Machar fled South Sudan in July.
Alexis Lamek has urged both sides to adhere to the fragile deal to finally end the civil war.
"It's actually terrible to see the conditions, the difficulty of this life. And we are all extremely shocked and horrified by the rapes and the violence which is continuing. And it just increases our resolve to ensure the peace agreement is fully implemented."
Samantha Power says that turmoil must be resolved.
"We have a lot of dire situations in the world. This is one of the worst. And we are prepared, as the international community, to continue to invest in standing with the people of South Sudan, but we need the government of South Sudan to do its part and allow the UN and the humanitarian community to be the partners that they really want to be."
