It appears to have been taken with the app Huji, a photo app that alters photos to give them a vintage look.
For the caption, the 26-year-old Gone Girl actress simply used a conch shell emoji.
The revealing snap has already racked up more than 860,000 likes on Instagram.
Along with this photo, Ratajkowski has also posted several other revealing snapshots. In addition to some recent low-cut dress pics taken in France, the model also shared a photo and video on boat in the middle of Paris.
In the video, which was posted in her Instagram Story, sees her driving the boat, while the pics shows her lounging around on the back of the vehicle.
"Still in Paris," she captioned the photo.
Ratajkowski has garnered a reputation for sharing revealing photos online, but she has always made sure that she does not do it based on expectations. In interviews, she has repeatedly said that she is simply doing what she wants.
"From the beginning, when I didn't have as much popularity, I made the decision to be as honest as I could about who I am and what I believe, and I have never apologised for that," Ratajkowski recently told Glamour Magazine. "I could have been like, 'I'll never post a sexy selfie again because I want people to take me seriously as an actress,' or I could have said, 'I'll never talk on political issues because I want people to see me as this sex symbol,' or whatever."
To Ratajkowski, her sexuality is a way of expressing feminism, despite what detractors say about her risqué shots.
"To start saying that certain people need to have a licence to be feminist is insane," Ratajkowski said. "Emma Watson said feminism isn't some kind of tool to beat other women with, it's supposed to be a freedom of choice. And I believe in sexuality. I think it's a wonderful thing and, if anything, I want women to understand their own sexuality outside of a patriarchal male gaze. We're the core of sexual beings, and I think that's something that should be celebrated rather than attacked."