Well, when your mixing with royalty you may as well dress like them, right?!
Wearing a dress worthy of a princess, Demi looked simply stunning in her beautiful cream frock with feather train.
And, heading to the Royal Variety recording at the London Palladium last night, the 'Neon Lights' singer joked her outfit was more than fitting for the occasion...
Sharing a photo of herself all glammed up, Demi tweeted: "My Ashi gown I'm wearing at tonight's Royal Variety. I love this dress! Ironically it makes me feel like a princess..."
We bet even Kate Middleton had a little wardrobe envy!
On arrival to the event, Demi made sure not to ruin her gorgeous number and was lucky enough to have a team on hand to carry her train as she waved to fans.
Heading inside, Demi joined the other celebs at the Royal Variety and, no doubt, rubbed shoulders with the evening's guests of honours, Prince William and Kate.
As well as her former X Factor USA boss Simon Cowell and his girlfriend Lauren Silverman.
Not only did Demi get to enjoy the star-studded evening out in the capital, but the 22-year-old also took to the stage to entertain the theatre audience.
Other performers included the likes of Britain's Got Talent winners Collabro, One Direction, Ellie Goulding and Ed Sheeran.
Demi's evening at the Royal Variety comes after she arrived in the UK earlier this week ahead of joining Enrique Iglesias on tour.
Set for a busy few weeks supporting the Spanish heartthrob on his 'Sex and Love' tour, Demi made sure to stop by Olly Murs first and join him for an acoustic performance of their track 'Up'.
Happy she paid a visit, Olly shared a cute photo of himself and Demi, telling fans he was glad he "finally got to hang out" with the US starlet.
Their collaboration features on Olly's new album, Never Been Better.
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Watch Demi's video for 'Neon Lights' here: