"For everyone asking…I have never been so hurt in my life," the 20-year-old wrote. "My heart has never been so broken…Lila my sweet baby girl has passed away."
She added: "Broken. Gonna go MIA for a bit. Need some healing time. Thank you to everyone who has sent love my way. I need it."
Earlier, Cyrus admitted that she was so upset she couldn't "think of one good reason to get out of bed today."
She has yet to announce the cause of her dog's death.
In October the singer begged for fans to pray for ailing two-year-old Lila. She tweeted: "Please pray for my baby girl Lila. She is in the hospital. I am so heart broken.
"Please send out vibes for her to not be afraid & to heal."
The former Disney star frequently tweets photos of her pet dogs who she calls her "babies." Her brood includes Floyd, Ziggy, Happy, Mary Jane and Penny Lane – a puppy that she adopted from a rescue shelter last month.
- Miley Cyrus’ Boyfriend Maxx Morando ‘Fuming’ Over Singer Fixation On Ex-Husband Liam Hemsworth: Sources
- Miley Cyrus Reached Out To Liam Hemsworth Before ‘Flowers’ Release, Ex-Husband Shut Down Attempts To Talk: Sources
- ‘Lost And Alone’ Miley Cyrus Leaning On Dolly Parton As Rift With Dad Billy Ray Worsens After His Engagement To 34-Year-Old Aussie Singer
On Friday she wrote on her blog: "It is with deep sadness that I have to tell you all that my kitten, Mercy, has passed away.
"My heart is completely broken."
Happy 20th Birthday Miley Cyrus! See Her Transformation From Disney Star To Controversial Young Adult
Cut Above The Rest! Liam Hemsworth Is Behind Miley Cyrus' Edgy New Look
Rockin' New 'Do? Miley Cyrus Shows Off Chopped, Bleached Hair