Ms Halliwell will meet the UN Secretary General, Kofi Annan, in New York.
Last week she travelled to Uganda to film a report for Comic Relief.
Ms Halliwell, who suffered a breast cancer scare when she was 18, also launched Breast Cancer Awareness Month this month, visiting inmates at Holloway Prison and sixth-formers at a London school to stress the dangers of the disease.
She also auctioned her costumes for a cancer charity.
Her appointment, follows a decision by the Tottenham footballer David Ginola to continue work done by Diana, Princess of Wales, to highlight the damage caused by land-mines.
"She will be a great ambassador for young people," Alex Marshall, the UN population fund's deputy director for information, said yesterday.
[Verse 2] British girl (hey) Got a show in London Won't you come around?
THE FORMER Spice Girl, Geri Halliwell, is to become a United Nations goodwill ambassador. The once scantily clad pop star will be an ambassador for the UN population fund to promote awareness of reproduction health issues in Britain.
She will also work with the Marie Stopes charity, which runs abortion clinics and advises on contraception.
Her daughter, Bluebell Madonna, was one of her bridesmaids. Halliwell's former love interests include Russell Brand, Henry Beckwith and David Walliams.
Comedian Dawn French was pictured going into the church. In 2005, while she was dating Sacha Gervasi, a screenwriter, she fell pregnant with her daughter.