The LOL actress, who only became engaged to the Hunger Games star earlier this month, hit back at "dumb" reports sparked by photos of her at a pool with friend Cheyne Thomas.
"I love my fiancé & he loves me," she wrote. "All the rest is bull s**t. We get to live a life of happiness no one can take that away from us.
"So now because I am engaged I can't have a friend of the opposite sex? can't have a friend help me out while I'm working alllll day? #dumb."
Addressing online critics who had been taken in by the story, Cyrus added: "Maybe YOU would have friends too if you'd get your face out the tabloids & start living your own life :) unfortunate."
Meanwhile, Thomas himself also criticised the speculation over his relationship with Cyrus, writing on Twitter: "Not sure if u guys realise it or not but you can hang out with ur friend and look happy and be having fun..there's nothing wrong with that.
"If that seems like strange behaviour to you then maybe u should reevaluate your friendships and quit worrying about mine... People should learn to be happy for others and not always try to ruin everything they're happy as hell let them have that."
Cyrus previously declared that her engagement to Hemsworth made it feel "like all [her] dreams are coming true".
The couple first met while filming The Last Song in 2009.