Willis, 22, sparked a debate on social media about Internet standards after posting photos of herself topless in public in New York City on her Twitter account on Tuesday, picking up support from the likes of pop star Rihanna.
Willis said she was inspired to confront Instagram's policy after topless photos Rihanna posted of herself on Facebook Inc's Instagram were taken down last month.
"This is about helping women feel empowered to make personal choices about their bodies not dictated by what society says is decent," Willis wrote on Twitter, which allows nude photos.
Willis, who posted a topless photo with the #FreeTheNipple hashtag campaign, said her Instagram account was suspended after she had posted "artistic" and "tasteful" nude photos.
Instagram said in a statement its policies are designed to strike a balance between creative expression and making the service a "fun and safe place."
"Our guidelines put limitations on nudity and mature content," Instagram said. "Once content is reported to us, we review it and remove if it violates our policies."
Willis said Instagram's policy was contradictory, allowing photos of near-naked women she believed were demeaning but banning any photos that showed a woman's nipple, including those of a woman breastfeeding a child.