The 53-year-old model first began working with the children's charity in 2011, and Heidi is thrilled to have been named an ambassador for the organisation.
Heidi told People: "I'm incredibly honoured by this recognition.
"After working closely with UNICEF for a while now, becoming a UNICEF USA Ambassador means so much to me because it deepens my commitment to helping children around the world. It's a privilege to use my voice to support UNICEF's mission and advocate for every child's right to a healthy, safe, and hopeful future."
Heidi - who has Leni, 22, Henry, 20, Johan, 19, and Lou, 16 - admitted that some of her past trips with UNICEF have changed her perspective of motherhood.
The TV star said: "When I travelled to India and Haiti with UNICEF, I met so many mothers who had been through unimaginable hardships. Seeing premature babies in Haiti wrapped in plastic bags under desk lamps was something I will never forget.
"It made me realise how much so many of us have, and how many children around the world still lack even the most basic necessities. As a mother, it reinforced something very simple: no matter where we come from, every parent wants the same thing: for their children to feel safe, loved and protected.
"What stayed with me most was seeing how much children respond to even the smallest acts of kindness. A smile, a hug, someone truly listening to them and playing with them, those moments can bring hope back into a child's world."
Heidi stressed that children "deserve to laugh, imagine and just be kids" regardless of their circumstances and where they're born in the world.
Heidi said: "June 11 is the International Day of Play, and I love that we celebrate something so simple, yet so essential. Play is not a luxury for children, it's a right.
"Through my work with UNICEF, I've seen how play can bring comfort and joy to children living through conflict and crisis. Even in the most difficult moments, children still deserve to laugh, imagine and just be kids."
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