"Wishing Archie a very happy first birthday today," the caption said.
Archie is Elizabeth's eighth great-grandchild and seventh in line to the throne. Harry and Markle got married in May 2018 and announced that they were expecting their first child in October.
Archie's last name of Mountbatten-Windsor goes back to 1960, when Elizabeth and Philip chose to make their own surname for their own direct descendants to be distinguished from the rest of the royal family. Mountbatten comes from Philip's surname, which he used while serving as a lieutenant in the Royal Navy.
For Archie's birthday, Markle read children's book Duck! Rabbit! Harry recorded the reading which was posted to the Save The Children U.K. Instagram page in support of the charity.