Read on for three things to know about Nauert.
Trump is downgrading the ambassador's position to a sub-Cabinet-level post. Since Haley announced her departure, the U.S. has become more aggressive in insisting budgets for United Nations peacekeeping operations in Africa be frozen or cut. In a sign of the difficulties the administration has had, the UN General Assembly on Thursday rejected a US -sponsored resolution condemning Hamas, the Islamist group that has ruled the Gaza Strip since 2007.
He added that colleagues described Nauert as a quick learner who "executed her functions capably". She cut back the once-daily State Department press briefings to around once a week.
But her relationship with current Secretary of State Mike Pompeo appears to have been better. "I have great confidence in her".
However Ms Haley has made no secret of the fact that she has had policy disagreements with Mr Trump.
"Heather Nauert has been at the State Department so it's not as if she has no experience".
"Nauert is a very good public operator and should do a professional job presenting USA policy at the UN".
In that speech in Brussels, Pompeo questioned the value and credibility of organizations like the United Nations, the European Union, the International Monetary Fund and World Bank, saying they are in need of dire change if they are to continue to have USA support or membership.
Newt Gingrich joined Bill Hemmer and Sandra Smith on "America's Newsroom" Friday to react to President Trump's picks for the next attorney general and United Nations ambassador.
Nauert, 48, is the acting under secretary of state for Public Diplomacy and Public Affairs.
Nauert's appointment would reinforce the link between Fox News and Trump, who has been accused of turning to the conservative-leaning channel, rather than experts, for policy advice.
In 2016, she travelled to battleground states to report on the presidential primaries and election. She also previously worked as a health insurance consultant in Washington, according to her State bio.
Nauert became an anchor on Fox & Friends, Trump's favourite television talkshow, where she helped air the network's recurrent themes about the threat of immigration, warning that immigrant children arriving in the U.S. could be bringing disease into the country, and that Sharia law was spreading in urban areas with Muslim populations.
Ms Nauert's perfectly young looks, good shape and the dazzling smile have long won her a fair bit of fame, even long before the blonde member of the Council of Foreign Relations and proud holder of a master's degree in journalism from Columbia University was readying to enter the White House in the role of State Department spokeswoman.