'Bad Guy' singer Eilish, 19, used to dress in baggy clothes but has been changing things up and appeared on the front cover of Vogue in a corset.
Speaking to Elle, she called out double standards in the industry as she revealed she had lost 100,000 followers when she showed off a different style.
Madonna, 63, also spoke out to defend the star, saying “we still live in a very sexist world” where women are put into categories.
“You’re either in the virgin category or the whore category," she said. "Billie started off in a non-sexualised category, not pandering to the masses and not using her sexuality in any way, which is her choice and God bless her for that - after all, she’s been a teenager all this time.
“[But] if she wants to turn around and take photographs where she is portrayed as a feminine woman, showing her body in a way that she hasn’t in the past, then why should she be punished for it?”
Madonna – who is known for being the queen of reinventing herself – said women should be able to portray themselves in any way they choose.
She also said that if Eilish was a man, “no one would be writing about this”.
Watch: Madonna stuns MTV Video Music Awards with surprise performance