"♡same dress but v different day♡" she captioned the gallery images, showing off her legs in an incredibly tight dress, with cleavage.
Fans immediately took to the comments section to scold the Stranger Things star for her revealing outfit.
"You're [sic] becoming [sic] more and more like the others 👎 you [sic] are 15... not 25... I really think you [sic] will be soon a very bad influence on children... so sad... as a celebrity you [sic] have to be responsible," one person commented.
"Am I the only person thinking she's a bit young to wear that kind of dress ?!"
"Cover your cleavage for real [sic]."
"Very inappropriate for your age," wrote another.
Some others were actually on Millie's side, claiming that it is up to her to decide what she wears, as it is her parents.
"I read through some of these comments. All I'll say is this, it's up to her parents and her to decide what's appropriate to wear. I fully understand what some of you are saying, but ultimately it's not your place to say. It's just a dress, some of you must not get out much."
This is not the first time the actress has been criticised for her inappropriate clothing.
In January this year, Millie shared a photo of herself in a skin-tight snakeskin dress with high heels. At the time, she was only 14-years-old.
At the time, Brown hit back at the negativity.
She wrote, "I know [sic] everyone on my last pic wants me to 'act my age' but quite frankly it's my Instagram and if I choose to post that picture and you don’t like it… scroll past it)."