The singer joined Bradley Walsh on stage as he hosted a Christmas extravaganza from The Hammersmith Apollo, which is set to air on ITV at 8pm Friday 26 December 2014 (Boxing Day, just in case you needed a reminder).
Katherine looked pretty as usual in a cream dress with an embellished neckline, a splash of red lipstick, and her signature wavy hairstyle.
She showed off mock-shock as Kermit sat in front of her - presumably being just a little bit cheeky.
The mezzo-soprano isn't the only star joining Bradley for the show - one of the hottest bands in the UK (cue lots of girls screaming), The Vamps, will also perform, and there's comedy from Al Murray, The Pub Landlord, as well as entertainment from the world-renowned Cirque de Soleil, who'll put on a show-stopping routine.
The cast of the hit West End musical, Irving Berlin's White Christmas will also be performing.
It could be the last time Katherine performs before enjoying her Christmas break, which she should make the most of as it sounds like next year could be very busy promoting her latest album Home Sweet Home.
Speaking to the Express, she said: "I'm concentrating on Home Sweet Home until Christmas and then my album comes out in America next year so there's a 12 month cycle of touring and promoting which is nice and I'm going to enjoy going back to places like South Africa and Asia, Germany."
LOVE KATHERINE? See more of the pretty star in our fashion gallery below: