The four images, taken by the duchess, show the little prince playing and posing on a beach near the family's home in Norfolk. Kate, a keen amateur photographer, normally takes the photos of the Cambridge children that are shared with the public.
Louis, whose full name is Prince Louis Arthur Charles of Cambridge, was born April 23, 2018. He is the third of William and Kate's children, preceded by big brother Prince George, 8, and Princess Charlotte, 6.
The location of the recently released photos appears to be the same as the beach shown in a family video that William and Kate released last year to celebrate their 10th anniversary.
Although the Cambridges initially only shared two photos of their youngest son with the public, after an enthusiastic response ("Prince Louis" was trending on Twitter on Friday), they shared two additional images from the same day to thank the public for their well wishes.
Louis's birthday is only one day after his great-grandmother Queen Elizabeth's birthday, April 21. To mark her 96th birthday, the royal family released a photo of the Queen with two Fell ponies named Bybeck Nightingale and Bybeck Katie.