Members of the public can now buy Harper's previously-worn outfits in the sale at the Mary's Living Save the Children Shop in Primrose Hill, or via the eBay store.
Beckham said: "I am delighted to be supporting the wonderful work of Save the Children through the Fashion Saves Lives Sale.
"As a mother, I passionately believe that all children, wherever they live, have the right to a happy, healthy life."
Not only has the fashion designer and mum-of-four donated a selection of items from her three-year-old daughter's wardrobe, she has also spent time carefully put outfits together with adorable matching shoes and jackets.
Like this one (one of our favourites):
Many of the items are designer, including pieces from Chloe, Roksanda Ilincic, Ralph Lauren, Stella McCartney, Gucci, Marc Jacobs and Charlotte Olympia.
Beckham added: "Everyone out there can do their part by purchasing or donating, ensuring children all around the world have the opportunity of a brighter future."
Fronted by Save the Children ambassadors Erin O’Connor and Mary Portas, the Fashion Saves Lives sale will also running on the Mary’s Living & Giving eBay store for Save the Children for a ten day period until 28 June.
The sale includes clothes from other celebrities including Melanie C, Fearne Cotton, Alex Gerrard, Rachel Stevens and Sam Bailey, according to Fashion Capital.
Save The Children is currently backing a campaign to ensure that at the UN summit in September world leaders make a commitment to end child deaths from preventable causes by 2030.
They found that 17,000 children die every day from preventable causes such as pneumonia and malaria due to a lack of adequate health services.
For more information and to purchase an item from the sale, follow the hashtag #FashionSavesLives on Twitter.
So now you want to see all the outfits, don't you? Browse through all 25 in the slideshow below.
But remember, they can't be purchased until 18 June!