Trump made the announcement Tuesday in a post on Truth Social, his social media website. He described Walker as "a successful businessman, philanthropist, former Heisman Trophy winner."
Walker, a Republican, tried to unseat Democratic Sen. Raphael Warnock two years ago with Trump's endorsement. Walker benefited during the campaign from nearly unmatched name recognition from his football career at the University of Georgia and in the NFL. But he fell short in his efforts, unable to overcome a bevy of damaging allegations, including claims that he paid for two former girlfriends' abortions despite supporting a national ban on the procedure.
He campaigned for Trump this year in his home state of Georgia.
To hold the ambassador position, Walker will need his nomination approved next year by the Republican-led Senate.
Trump also announced Tuesday that he was nominating art collector Nicole McGraw as the U.S. ambassador to Croatia.
Trump has chosen a variety of loyalists to diplomatic posts. Charles Kushner, the father of Trump's son-in-law Jared Kushner, was named ambassador to France, and Rep. Elise Stefanik of New York, one of his strongest defenders in the House, was picked as his ambassador to the United Nations.
He selected another Georgian, former Sen. David Perdue, to serve as U.S. ambassador to China. Perdue lost his Senate seat to Democrat Jon Ossoff four years ago and ran unsuccessfully in a 2022 primary against Georgia Gov. Brian Kemp.
Another former senator from Georgia, Kelly Loeffler, was also tapped by Trump for a job. He chose her to lead the Small Business Administration.