Kelly, who will be a judge on this season's show, stunned in a grey tailored crop top with a plunging neckline and a matching long skirt with a split through the middle.
All eyes were on the Destiny's Child star as she gave interviews to the surrounding press on the special blue carpet.
Kelly perfected her look by rocking sleek shoulder-length hair and a slick of pink lipstick.
Meanwhile, fellow judge and 'Heart Attack' singer Demi opted for an all-black outfit as she attended the bash at Cecconi's West Hollywood.
The 21-year-old, who is set to appear in the new season of Glee, looked sophisticated as she walked onto the blue carpet wearing a tailored suit jacket with shoulder pads.
Looking fierce as she posed for photographers, Demi completed her look with cropped trousers and black heels.
Third judge, Mexican singer Paulina Rubio, also stunned in a white jewelled mini dress, and made a heart symbol with her hands in the air. The ladies certainly dressed to impress last night!
Music mogul and judge Simon Cowell also graced the event. The expectant father was all smiles as he wore black glasses and his trademark white shirt and jeans.
But equally happy was the show's host Mario Lopez, who was supported by his pregnant wife, Courtney Laine Mazza. Mario, who is set to become a dad for the second time, beamed from ear-to-ear and cradled Courtney's baby bump. Aww!
The X Factor USA airs in the US on 11 September on FOX. It is expected to air on ITV2 in the UK at a later date.
Watch a video of the X Factor judges in action below: