The 20-year-old posted a picture on microblogging website Twitter with the caption: "Because I am her favourite and she is mine."
Cyrus confirmed that the tattoo is of her "mammie" after a fan asked who the portrait was of, reports a website.
The tattoo, which is on the 'Hannah Montana' star's right forearm, shows her grandmother's face and neck and fades just before getting to her wrist.
The tattoo was made by celebrity tattoo artist Kat Von D.
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