"Happy happy Birthday dear Beabea," Eugenie captioned.
Referencing the first picture, Eugenie wrote: "Here we are on the eve of your wedding. Still rocking a scrunchie or hair accessory (since the 90s) and huge smiles on our faces."
In the second pic, which shows the sisters as toddlers standing next to their mother Sarah Ferguson, Beatrice is wearing a scrunchie that’s very similar to the one she wore in her pre-wedding snap.
"Such a beautiful shot of two sisters, no makeup, no nonsense just sibling love," wrote one fan referencing the first image.
Another quipped of the second archived shot: "Ah, back in the days where smiling just meant showing as many of your teeth as possible! Happy birthday to HRH."
Beatrice's mum Sarah also took to the social platform to share her own birthday message for her eldest daughter.
"Happy Happy Birthday My Dearest Beatrice. 8.8.88 at 8.18pm was one of the greatest and most proud moments of my life. You are unique and simply one of the most special people on this planet. Your ever loving and devoted Mum," Sarah wrote.
A beautiful image of Beatrice as a young girl smelling a pink flower was included in the birthday tribute.
Buckingham Palace also shared a candid image of Beatrice having a chuckle with the Queen in an additional birthday tribute, which was shared on the official Instagram page.
"Wishing Princess Beatrice a very Happy Birthday!" the Palace wrote, before adding: "Her Royal Highness is The Queen's 5th grandchild."
"This [picture] was taken when The Princess joined her grandmother at a Maundy Thursday Service at York Minster."
The touching birthday tributes come after Beatrice tied the knot with hubby Edoardo Mapelli Mozzi in a top secret ceremony in Windsor in July.
Given the COVID-19 restrictions, the service was a much smaller affair than originally planned, with only 20 or so guests attending the socially-distanced ceremony and celebration.
Among those who attended the intimate affair was the Queen herself, who was pictured with the newlyweds alongside her husband Prince Philip after the ceremony.