The Hollywood actress will dance and sing along to the iconic J track 'Get Happy' on the show's special 100th episode today (23.07.09) whilst wearing a skimpy fedora, nylon stockings and a figure-hugging skirt.
She said: "This is an homage to Judy Garland and I am very excited about it. I like to sing and dance so it's really fun."
The 30-year-old beauty - who has a three-year-old daughter Suri with husband Tom Cruise -says she became involved with the show to help raise the profile of children's charity Dizzy Feet Foundation.
She explained: "It gives scholarships to these young kids. I think it is important to experience music and dancing and singing and the arts and sports. This is the next generation. I think it is important to give them every opportunity to grow and discover what it is that they like and have access to the best."
The charity was founded by the show's producer and judge Nigel Lythgoe, judge Carrie Ann Inaba and director and choreographer Adam Shankman.
Tyce Diorio choreographed the routine which will see Katie thrust into the air over her backing dancer's heads in tribute to the 1950s musical movie 'Summer Stock'.