Bob Geldof's eldest daughter showed off a sizable diamond on Instagram with the caption: "Boydonegood".
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Fifi shows off the ring that sealed the deal, writing: "Boydonegood"
The boy in question is her 33-year-old fiancé Andrew Robertson, who describes himself as an "artist and busker", and makes beach sand sculptures.
There's been no confirmation on when the wedding will take place, though a source told the Daily Mail it would go ahead in July next year.
Fifi isn't the only Geldof family member set to walk down the aisle. Last week it emerged that Bob is to wed his long-term partner, French actress Jeanne Marine.
Friend of the couple Jerry Hall confirmed the happy news. "Bob has proposed," Jerry told the Daily Mail. "She is over the moon. Jeanne Marine has been a rock and a tower of strength over this. I'm so excited for her I am bursting to tell someone.
Peaches Geldof's elder sister will wed artist Andy Robertson
"Bob and Jeanne have been through hell, but they are determined something positive will come out of it. After all the sadness, we need something to focus on which will bring joy and light.
"I've just got back from Texas and accompanied Jeanne to the funeral," she continued. "It was, of course, terribly sad, but she told me that Bob had asked her to marry him out of the blue.
"I'm so happy for her. It makes you realise we all have to value what we have."
Fifi posted this "sunny sideways selfie" along with the photo of her ring
Jerry made the comments on the same day an inquest confirmed that Peaches' death was "likely" to have been caused by heroin.
Police found no evidence of drug paraphernalia in the house, leading to speculation that it may have been removed before their arrival.