"I'm not engaged," she announced to her 5.3 million Twitter fans.
"I've worn this same ring on this finger since November! People just wanna find something to talk about! It's a topaz people!"
The wedding rumor began late last week when Cyrus tweeted a picture of her left hand sporting what appeared to be a solitaire diamond engagement ring.
She sent the photo to show off a new manicure, in promotion of a brand of nail polish.
Hemsworth, 22, and Cyrus, 19, have been dating since meeting on the set of their 2009 film "The Last Song."
Until recently, Hemsworth had been known mostly for his relationship with the former "Hannah Montana" starlet and as the younger brother of "Thor" actor Chris Hemsworth.
But the younger Hemsworth's star is on the rise thanks to a leading role in the blockbuster film "The Hunger Games," which had the third-biggest US opening ever this weekend, raking in an estimated $155 million.