"Everybody in her circle knows they've been together this whole time," a source told the mag.
Another source said the 21-year-old twerking enthusiast and Mike Will, real name Michael Len Williams II, are "pretty serious" and that mom Tish Cyrus believes he's "part of the family."
Mike Will, who's previously worked with Rihanna, Lil Wayne and Ciara, served as executive producer on the "Hannah Montana" alum's fourth studio album and wrote and produced its first single, "We Can't Stop." Cyrus was also featured on his debut song and album, "23."
However, romance rumors first cropped up about the couple in October, when she was reportedly grinding and kissing the 25-year-old hit-maker during her "Bangerz" album release party in Manhattan, Us Weekly reported at the time.
Shortly after those reports surfaced, the onesie-loving "Wrecking Ball" chanteuse told Ellen DeGeneres the album was primarily about her relationship with Hemsworth and that she hadn't been dating and was pretty happy about it. (Well, some wouldn't consider grinding or kissing to be actual "dating.")
"I really haven't gotten to see anyone right now. 'Cause I've been like so crazy busy. I'm just letting that chapter kind of close and just looking forward to that new one," Cyrus told DeGeneres.
"I want to be really clear and determined with everything I'm doing in my life right now, and I have been. ... I keep saying I'm the happiest I've ever been, and that's not even a dig," the singer added.
"That's just me because I really am genuinely happy finally with myself, which I think sometimes it takes you separating yourself from someone else to really be happy with who you are. I've never been able to just be alone, and I'm obsessed with being alone and like hearing my thoughts."