The new official account for the Duke and Duchess of Sussex - AKA Harry and Meghan - posted a comment on the pic.
Expecting their first child any day now, Prince Harry and Meghan Markle were also quick to congratulate the young prince. The pink-cheeked youngster's blonde hair is carefully combed to the side as he crawls about a lush green space.
Few photos of the youngest prince have been made public.
This photo released by Kensington Palace on Monday April 22, 2019, and taken by Kate, Duchess of Cambridge, shows Prince Louis at their home in Norfolk, England, to mark his first birthday on Tuesday. Last November, in a series of adorable photographs taken to commemorate Prince Charles' the Prince of Wales' 70th birthday, Prince William's father takes huge joy in holding Prince Louis, as the then-6-month-old babe smiles radiantly in his grandfather's arms. Middleton is often credited as the official family photographer, having taken numerous portraits of Prince George, Princess Charlotte and now Prince Louis that have been shared with the rest of the world.
The Duchess of Cambridge, 37, was the photographer behind Charlotte's first birthday portraits, too.
Royal author Phil Dampier told the Sun: "It starts to look like Harry and Meghan are in competition and are out to steal Louis' thunder".
Prince Louis Arthur Charles arrived at 11.01am, weighing 8lb 7oz, on April 23 2018 - patriotic St George's Day.
To mark his first birthday, three official photos of the tot were shared to the Palace's Twitter page, showing little Louis smiling in the grounds of the family's home on the Queen's Sandringham Estate in Norfolk.
Another person wrote: "Louis is a mix of George and Charlotte".
During a visit to a children's centre, Kate said: "He just wants to pull himself up all the time".