The actors had been fending off breakup rumors for months. In June, Cyrus, 17, denied they had split, telling fans on her Web site that they were "so happy."
"I'm not really a big lovey person. I'm not, like, mushy. [But] Liam was the first person that made me want to be in a real relationship," she told PEOPLE in April.
For his part, the Australian actor, 20, called their relationship "pretty serious" and said he admired her "big heart" and how she was "just so open."
The couple's split was first reported Wednesday by Gossip Cop.