Wrecking Ball singer Miley showed off the new body art – a black and white portrait – on her Twitter page. In the caption, she wrote: "Because I am her favorite & she is mine".
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It was done by Miley's favourite tattoo artist Kat Von D, who specialises in tattoos of this kind.
Kat worked from an old photo of Miley's gran, which appeared to have been taken when Loretta was about the same age as 20-year-old Miley is now.
A day later, Miley tweeted: "Yesterday was a day of making music, tattooing, and hangin w friends... Gonna repeat that today!"
Time will tell if she is adding another tattoo to her collection, which currently numbers 19 designs.
Other inkings include a dream catcher on her side, the word "love" on the inside of her ear and a quote from Theodore Roosevelt on her forearm.
It reads: "So that his place shall never be with those cold and timid souls who neither know victory nor defeat."
She got her first tattoo aged 17 – the words "just breathe" written across her ribcage.