Watson, who landed the role of the precocious Hermione Granger aged nine, turned 31 yesterday (15 April).
Felton accompanied the photo of himself and Watson as young teenagers with the caption: “Happy birthday to the one & only.”
The post, which has since received over 2.4 million likes, has attracted attention from fans commenting on how “cute” the photo is.
“This is too cute,” wrote one user, while another added: “I’m gonna cry.”
A third person commented: “You two are so adorable together.”
Last year, Harry Potter fans were shocked after discovering Felton’s total screen-time across the franchise.
According to a resurfaced rundown of the different characters’ screen time on IMDb, the recurring antagonist tallied just 31 minutes over the entire saga despite being considered a main character.
In fact, Felton only features on screen in Harry Potter and the Order of the Phoenix for one minute and 15 seconds.