The LOL star, 19, met the Hunger Games actor on the set of her film The Last Song in 2009.
In a statement, the singer told People: "I'm so happy to be engaged and look forward to a life of happiness with Liam."
The actress previously sparked rumours of her engagement in late March after tweeting pictures of a large diamond ring on her left hand.
Cyrus is currently working on her new album, while her new fiancé completes production on Empire State in New Orleans.
Watch the trailer for LOL below: