"You are exceptional.. Happy Birthday my love," Eugenie penned with heart eye, celebration and red heart emojis.
The post included three images of Jack enjoying the great outdoors with baby August. The first photo showed the new dad looking into the camera with his son on his shoulder. The nearly-three-month-old wore a personalized knitted cardigan from Uncool Wool with "August" embroidered on the back and a matching white hat with star print. Jack was also in a white shirt.
The second shot captured Jack grinning at the camera from the side as he held August in a baby carrier. The final image showed the birthday boy walking by himself in a gorgeous garden. He had his back to the camera and he was strolling on a scenic river path filled with white and yellow daffodils on a gloriously sunny day.
It has not been revealed where the photos were taken, but the young family is believed to have settled in at Frogmore Cottage in Windsor after welcoming August.
For her 31st birthday, the younger Princess of York posted a smiling shot of her, Jack and August to thank followers for their birthday wishes. The little boy was wearing the same personalized knit sweater in the image as he was in the latest uploads.
"Thank you for the birthday love yesterday," she wrote on March 24 with a red heart emoji. "I got the best present I could ask for!!"
"I'm so excited to be August's mum and as you can see I'm enjoying my first Mother's Day," she wrote on Instagram on March 14, next to a sweet snap of the little guy in honour of Mother's Day in the U.K.
The sweet images captured August wearing killer whale pants, a white knit sweater and bunny slippers surrounded by a garden filled with daffodils.
Jack and Eugenie welcomed their baby boy on Feb. 9 at Portland Hospital in London. Almost two weeks later, the new parents officially introduced their son and revealed his name, August Philip Hawke Brooksbank.
"Her Royal Highness Princess Eugenie and Mr Jack Brooksbank have announced that they have named their son August Philip Hawke Brooksbank," read the caption on the @TheRoyalFamily Instagram next to the first full photos of August.
The announcement continued, "The couple have been touched by the well wishes they have received on the birth of their first child, and are delighted to share this first photograph as a family.
"The photograph was taken by their midwife during a home visit."