Cyrus, 23, and Song, 24, announced their engagement in October.
"Brenda and I have decided to go our separate ways," Cyrus said in a statement to People.com. "We split up a couple of months ago. We will continue to focus on our careers. I wish Brenda the best and much success in the future. I hope everyone can respect our privacy about this situation."
Song is a former star of the sitcom "The Suite Life of Zack and Cody." She also played a supporting role in the film "The Social Network."
Cyrus' younger sister, singer/actress Miley, announced her engagement to actor Liam Hemsworth last week.