Kate Middleton kicked off Christmas on the keys and on key ... for the first time ever, she played the piano publicly ... backing up Tom Walker's Xmas performance.
The Duchess of Cambridge showed off some pretty serious skills as she played, "For Those Who Can't Be Here" at Westminster Abbey ... which, BTW, looked awesome.
The vid was released Friday, though it was recorded a few days back at a royal Christmas carol service called "Together at Christmas" -- which Kate hosted. She had tweeted... "I’m so excited to be hosting #TogetheratChristmas here at @wabbey, a place that’s really special to William and me."
Tom raved about the night they shared the stage ... "WHAT A NIGHT🤯 I can’t thank enough for being a part of this performance. This is dedicated to those who can't be here this Christmas including my grandad."
You gotta see the video just to see how amazing Westminster Abby looked.