The elaborate design depicts the Egyptian goddess Isis and is in honour of her late grandmother, who passed away in July.
The 'We Found Love' singer, who is currently in London after performing at the Paralympic Closing Ceremony last night, tweeted the first photo of her 18th tattoo with the message:
"Goddess Isis - Complete Woman - Model for future generations- #GRANGRANDOLLY - always in and on my heart #1lo"
Rihanna was very close to her grandmother, Dolly, and after her death in July uploaded photos of the two together to her Instagram.
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