The news comes after months of speculation that Cyrus' changing look and Hemsworth's rumored infidelity had taken a toll on the relationship.
Sources told E! News that the 20-year-old "Wrecking Ball" singer had initiated the break up "after coming to grips with Liam being what she believed was less than faithful to her."
Rumors surfaced last week that the 23-year-old singer had been sexting "Mad Men" actress January Jones. He was also seen leaving an Oscars party with her last February.
Cyrus opened up about her wedding plans in a Harper's Bazaar interview published the same day her rep confirmed the break up. She said her busy career made it hard for her to plan a wedding.
"I definitely don't have time to deal with a wedding right now. But I will at some point," she said.
The two met filming "The Last Song" in 2010 and became engaged in 2012.