The actress, 44, wowed in a figure hugging white frock with a large side split up one side and a cross-over cut exposing her chest.
She showed off her impressive physique in the ensemble which emphasised her petite frame.
Naomi even gave a hint of her toned legs as well as she put her left leg on display through the split and wore a pair of black high heels.
The star wore large earrings and a simple bracelet around her right wrist, but she kept her hair 'do simple.
Watts opted for an up 'do for the occasion with her long fringe hanging loose.
Naomi revealed at the premiere that she wanted to play Princess Diana because she was such a "complex" woman.
The 44-year-old actress portrays the late royal in new movie Diana - which is based on the book Diana: Her Last Love and focuses on the last two years of her life including her romance with Dr. Hasnat Khan.
Naomi initially turned down the part twice but was compelled to take on the role because she found Diana - the mother of British Princes William and Harry - so fascinating.
Speaking at the world premiere of the film at the Odeon cinema in London's Leicester Square tonight, she said: 'Any woman I take on (on screen) I want them to be all things.
The production have done well to recreate classic images of Diana throughout the film, focusing on the last two years of her life as she continued her charity work, supporting AIDS victims and campaigning against landmines with the Red Cross.
But it was her little known relationship with a heart surgeon that grips our attention as the truth behind their failed union comes out in the trailer.
At first, Diana's attraction to Khan stems from his treatment of her as a normal person, with her saying to a friend: 'He doesn't treat me like a princess, it's almost like he doesn't know who I am.'