The 23-year-old singer bit into a bagel and one of her gnashers fell out, just three hours before she had to get into her bridal gown.
'My tooth fell out in my hand. Thank god I didn't swallow it. Not good for a bride on her wedding day,' she told US chat show host Ellen DeGeneres.
'I bit into a bagel. I was sitting on the bed with my mum and my sister. It was 11 in the morning. We started getting ready at two.
'I burst into tears, and I started laughing at myself, then I really burst into tears.
'My sister said, 'Go and get into the bathtub, don't get your face all red and ridiculous. We're never going to let you get married without a tooth.'
Duff, who rose to fame in Disney TV series Lizzie McGuire, eventually saw the funny side and sent a photo of her gap-toothed grin to husband-to-be Mike Comrie.