"I'm working on getting [my butt] back," she tells PEOPLE. "It used to be my favorite body part, but now it's disappeared!"
Luckily, the singer's got plenty of techniques in mind for the task.
"I'm going to have to start hiking or at least going on the elliptical or walking on an incremental treadmill or horseback riding," she says. "Something that firms the butt."
And while her body looks firm and tight in her steamy underwear ads, she's hardly starving herself to achieve that enviable figure.
"I'm eating everything," she says. "I've been eating ice cream and fast food and Italian food."
But the singer believes the way that she hydrates plays a role too. "I drink a lot of coconut water," says Rihanna, who is the face of Vita Coco. "It balances out all the other toxic stuff I put into my body."