Kate appeared alongside the musician Tom Walker in a clip shown at the Westminster Abbey event.
The pair took part in secret filming at a recording studio ahead of the concert, which was broadcast on ITV.
Royal Carols: Together At Christmas was staged for people who supported their communities during the Covid pandemic.
A clip of the duchess performing with Walker on his previously unheard Christmas song, For Those Who Can't Be Here, has been posted on the Kensington Palace Twitter feed.
