Mum Victoria quickly followed suit, sharing an adorable snap of her daughter embellished with a snapchat flower filter.
"Happy Birthday to the little girl with the biggest heart 💕 kisses from mummy, daddy and your big brothers x 💗💗💗💗 #HarperSeven," the VB designer wrote.
Harper's big brothers got in on the action, with 16-year-old Romeo sharing a gorgeous snap of the two with the words: "Happy birthday to the best sister in the world I can't believe you're 8!!! Love you so much and hope you have the best day ever."
Cruz, 14, posted a photo of his baby sister playing the electric guitar with the caption: "HARPER happy birthday I love you so much you are such an amazing little girl. Have an amazing day Harper."
Eldest brother Brooklyn, 20, chose a photo of Harper cradling a puppy and gushed: ""Happy birthday to the best sister ever xx I love you harper so much. Have the best day ever."
Harper's birthday comes amid reports that Brooklyn is struggling in his elite photography internship with acclaimed British photographer Rankin.
Sources say the model and aspiring snapper has found even the simplest tasks difficult and lacks basic knowledge on how to work a camera.
"Everyone knew Brooklyn's work needed fine tuning, but no one knew his knowledge of the simplest tasks was so off," an insider told The Sun.
"Everyone had high hopes for him but it's not been the most impressive start. He's lacking in most areas, but he is trying to make up for it in enthusiasm."
Victoria and David's first child has found himself in headlines many times this year, most notably during Cannes Film Festival when he and his model girlfriend Hana Cross were "separated by security guards" during an explosive row.
Sources say the Beckhams are increasingly worried about their son's "toxic relationship" with the British model, News.com.au reports.