Fans were quick to wish Jack a happy birthday in the comments section.
"Happy birthday Jack have a great day with your cute family," one penned in the comments.
"Looks a very happy daddy," remarked another.
"Happy Birthday Jack how lovely to have your photograph taken with your baby son August on your special day.
This is only the fifth time Princess Eugenie has shared a look at baby August since his birth in February.
The last time the mum-of-one shared a family photo was to celebrate Easter back in April when she shared a glorious snap of herself, Jack and August standing beneath a large blossoming tree.
"Wishing everyone a Happy Easter and a blossoming Spring. #happyeaster #blossom #spring," she captioned her post.
The happy photos come shortly after Eugenie and Jack attended the funeral of Eugenie's grandfather, Prince Philip last month.
Due to COVID restrictions, the event was attended only by immediate family and close friends of the Duke of Edinburgh.
In a sad moment, the youngest daughter of Prince Andrew and Sarah Ferguson could be seen holding on to a handkerchief.
"Dearest Grandpa, We all miss you.," Eugenie wrote in a heartbreaking post following news of his passing.
"You would be so touched by all the tributes that have been shared with me the past few days. People remember sitting next to you at a dinner, or shaking your hand once, who remember you saying hello in passing, or remember how much their DofE award meant to them.
"I remember learning how to cook, how to paint, what to read. I remember laughing at your jokes and asking about your spectacular life and service in the navy.
I remember incinerating the sausages and you swooping in to save the day. I remember your hands and your laugh and your favourite beer. I will remember you in your children, your grandchildren and great grandchildren.
"Thank you for your dedication and love for us all and especially Granny, who we will look after for you. With all my love Eugenie."