“Heather Nauert will be nominated,” Trump said on Friday, before departing the White House for an event in Kansas City.
“She is very talented, very smart, very quick and I think that she will be respected by all.”
Current envoy Nikki Haley announced in October she would be leaving the UN post at the end of the year.
The 48-year-old Nauert, whose nomination would require Senate confirmation, became the State Department’s spokeswoman in April 2017 and was named earlier this year as the acting undersecretary for public diplomacy and public affairs.
She has been touted for the post since November.
Trump told reporters last month the former Fox News anchor was “under very serious consideration”.
He added: “She’s excellent, she’s been with us a long time, she’s been a supporter for a long time, really – excellent.”
The State Department declined to comment and Nauert did not immediately respond to a request for comment.
James Warren, the executive editor of NewsGuard, said Nauert was an “unconventional” pick.
“When one looks at the history of UN ambassadors going back to the 1950s, you had people like Adlai Stevenson, who was the Democratic Party nominee for president twice, George HW Bush. You had rather prominent public intellectuals like Daniel Patrick Moynihan, the US Senator for New York, and a small group of women, who include Madeline Albright and Nikki Haley … So on that level, Nauert’s pick is notable,” he told Al Jazeera.
“On another level, it’s easily explained by Trump’s fascination with American cable news, and in particular with Fox Cable News Network,” the former Washington bureau chief for the Chicago Tribune said.
“And one of his favourite shows is the popular morning show, Fox and Friends, and Nauert was associated with this show for several years, and that surely had something to do with her selection.”
The New York Times said Nauert is just the latest Fox News personality to be recruited or elevated by Trump.
He had previously named John Bolton, a former UN ambassador who was a Fox contributor, as his national security adviser and Bill Shine, a former Fox co-president as his deputy White House chief of staff in charge of communications, the newspaper reported.
Nauert will face a variety of challenges if confirmed for the job, including championing the US efforts to contain Iran‘s influence in the Middle East and ensuring the global body maintains tough sanctions on North Korea, as Washington tries to negotiate an end to Pyongyang’s nuclear and missile programs.
Trump has been critical of the UN, complaining about its cost to Washington and criticising it for focusing on bureaucracy and process rather than results.
He pulled the US out of the UN human rights body in September, citing bias against Israel. His administration has also cut funding to the UN agency for Palestinian refugees, while last year it proposed to cut US funds for aid and diplomacy, which could curb the work of the global body.
But Trump has also used the UN to try to advance his foreign policy agenda on Iran and North Korea.