Spears, 31, wore a floral form-fitting leather dress with sheer panelling at the sides, while Cowell, whose face was looking slightly swollen, kept it casual in a black jumper and jeans.
The pair were joined by fellow judges Demi Lovato, in a floral dress and biker jacket, and LA Reid, in a grey suit and pink shirt.
Co-hosts Mario Lopez, with wife Courtney Mazza, and Khloe Kardashian, looking chic in a green skirt and silk blouse, also graced the red carpet.
The four final acts attending the party included Tate Stevens, Fifth Harmony, Carly Rose Sonenclar and Emblem 3.
Take a look at last weekend's X Factor USA show here: