The black-and-white image is the first glimpse the world has had of the Beckhams' daughter, who was born last week by caesarean at Cedars-Sinai hospital in Los Angeles.
LA Galaxy star David later followed up by posting a photo of Harper and her mother with the caption "I took this picture of my two girls sleeping".
The former England football captain wore pink-and-white boots with the names of all four children to last night's LA Galaxy game against Real Madrid.
During the game Victoria posted a Twitter message saying: “We all feel so blessed and they boys love their baby sister so much!!!”
Harper's first week has been documented extensively online, from her father's first announcement on Facebook.
He said: "I am so proud and excited to announce the birth of our daughter Harper Seven Beckham.
"She weighed a healthy 7lbs 10oz and arrived at 7.55 this morning, here in LA. Victoria is doing really well and her brothers are delighted to have a baby sister xx