The 'Heart Attack' hitmaker surprised fans on the opening night of her 'Neon Lights' tour in Vancouver when she performed with the former Jonas Brothers singer.
The duo belted out three tracks, 'Stop The World', 'Catch Me' and 'Here We Go Again' together, while Nick, 21, played the guitar.
The 21-year-old beauty - who has famously dated Nick's brother Joe Jonas in the past - had originally hired the hunky star as the musical and creative director of her tour a few months ago.
Nick said previously: ''I'm overseeing video content, wardrobe, lighting and staging. And then I'm extending into the musical side of things, which includes creating the arrangements for the songs.
''I'm building what Demi wanted, which is a show without stops and starts.''
The performer - who was previously in a band with his brothers Kevin, 26, and Joe, 24 - has been close friends with Demi for a number of years and wanted to make sure he did her proud.
He continued: ''I was immediately excited when I was offered the position. The relationship that Demi and I had over eight years really gives me the insight into how to best communicate with her and her team.''
As they do every summer, the Martin family relocate to their summer home over the...