Madison Lambe, 12, asked the Prince for a photo but was too stunned to press the buttons on her phone, so William took the snap.
"I couldn’t believe it when William came over to me," she said. "I asked him for a picture and he said, ’You can’t beat a good selfie on Christmas Day.'"
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"He was really nice and I was so excited," she added.
Madison had been waiting to catch a glimpse of William, his wife Kate Middleton and other royals as they walked to church at Sandringham on Christmas Day.
She had travelled with her mum Paula Scrupps, 38, from their home in Boston, Lincolnshire on Christmas morning.
It’s not the first time that Prince William has taken a selfie – he posed for a picture with an unnamed boy during a visit to the Coptic Orthodox Church in Stevanage, Hertfordshire last month.