"I don't have the CW girl body," she told the publication in an article released this week.
"Tiny waist, nice-shaped legs, skinny, small, tiny.
"I had to do a bra and underwear scene in this last season and I felt really insecure about it," said the 23-year-old actress.
"I really, really didn't want to do it. I didn't tell anyone this. I wasn't pressured into doing it. I did it because it was my job."
The Riverdale star added that she "felt bad about (her) self" while doing the scene, but ultimately felt it was important for her to do it as it would send the right message to fans.
She admitted she has a "complicated" relationship with body positivity.
"This is where it gets complicated. I can't preach body positivity if I don't practice it," said Reinhart.
"So even if I'm not feeling amazing about my body, I felt it was important for me to do the scene anyway in my bra and underwear so people could see my body as it was. I did it for the people who feel like they need to look a certain way."
Reinhart has been open about revealing scenes in the past, telling Twitter followers in February she feels "intimidated by the physique of (her) surrounding cast mates sometimes".
She said she felt it was her "obligation" to show confidence in herself in the fight to stamp out unrealistic expectations of women's bodies in Hollywood.
"I have bigger boobs, I have cellulite on my thighs/butt, and my stomach sticks out rather than curves in," she tweeted.
"I want other young women to see my body on TV and feel comfort in the fact that I'm not a size 0."
Reinhart was until recently in a relationship with Riverdale co-star Cole Sprouse.
Sharing the news of their split on Instagram, Sprouse, 28, wrote: "Lili and I initially separated in January of this year, deciding to more permanently split in March. What an incredible experience I had, I'll always feel lucky and cherish that I had the chance to fall in love."
He concluded by wishing her "nothing but the utmost love and happiness moving forward" before plugging her upcoming movie, Chemical Hearts.