She says her first ever relationship was with another female but that she's never enjoyed "acting like a girl" herself.
The 23-year-old was in a relationship with actor Liam Hemsworth between 2012 and 2013.
Pansexual people are attracted to others regardless of their sex or gender.
"My whole life, I didn't understand my own gender and my own sexuality," she tells US magazine Variety.
In 2015 she said she was "open to every single thing".
"I always hated the word 'bisexual' because that's even putting me in a box. I don't ever think about someone being a boy or someone being a girl.
"My eyes started opening in the fifth or sixth grade. My first relationship in my life was with a chick."
She's been celebrated and criticised for her onstage persona - but she says she never saw anything sexual in her performances or costumes.
"My nipple pasties and **** never felt sexualised to me," she adds. Miley says her parents didn't "understand" her sexuality when she rejected stereotypical female activities as a child.
"I know some girls that love getting their nails done. I hated it. My nails look like ****. I don't wax my eyebrows," she says.
"I never related to loving being a girl. And then, being a boy didn't sound fun to me."
She also reveals that she once sought help from an LGBTQ centre in LA, where she met a gender-neutral person who helped her make sense of her own sexuality.
"I feel very neutral. I think that was the first gender-neutral person I'd ever met," she explains.
"Once I understood my gender more, which was unassigned, then I understood my sexuality more.
"I was like 'Oh — that's why I don't feel straight and I don't feel gay.' It's because I'm not."
Miley also claims she has found a "purpose" in speaking out for LGBTQ+ rights and working for her foundation, Happy Hippie, which helps vulnerable LGBTQ people.
"On my tombstone, I didn't want the Wrecking Ball lyrics. I wanted it to be something greater," she says.
"I'm the only Disney star who would say I'm pro-lesbian and gay, before it was OK to say that."