"I don't wanna say it because I don't want it to be real... But my precious baby Floyd has passed away," she wrote. "I am broken."
She added: "I know I don't mean it but I wish he would've taken me with him this is unbearable. What am I gonna do without him?"
She first introduced fans to Floyd in July 2011, writing on Twitter, "I'm so in love with him it is ridiculous."
It's the second loss in the past two years for Cyrus, whose dog Lila passed away in December 2012 after she was accidentally killed by her pup Ziggy (who was later adopted out to another family).
The avid dog lover – whose canine brood also includes rescues Bean, Mary Jane and Happy – is currently on tour promoting Bangerz.
"I'm sorry in advance if I am not myself tomorrow Boston. I will try my best to be my best," she later Tweeted. "I am miserable."
I don't wanna say it because I don't want it to be real... But my precious baby Floyd has passed away I am broken pic.twitter.com/QBPujurnUj— Miley Ray Cyrus (@MileyCyrus) April 2, 2014