Australian Naomi, 44, plays Diana, Princess of Wales in the new biopic, which opens in cinemas on 20 September.
The slim star looked stunning in the figure-hugging floor-length white dress by Versace, which featured a crossover halterneck with a keyhole cutout, and a thigh high split.
(Princess Diana famously wore a strikingly similar white Versace dress on the front cover of Vanity Fair in July 1997, shot by photographer-to-the-stars, Mario Testino, just weeks before the Princess's death.)
Naomi chose a pair of simple black peeptoe platform heels, a delicate diamond bracelet and a pair of dazzling chandelier earrings, keeping her accessories simple so as not to upstage the scene-stealing dress.
With her blonde hair swept into a simple up-do with a side-parting, the star struck a chic and sophisticated note for the hotly awaited red carpet appearance.
After the premiere, Naomi Watts changed into another stunning dress for the after-party at exclusive members' club, 5 Cavendish Square.
Keeping her statement earrings and bracelet on, the Diana star switched into a black long-sleeved mini dress with sheer mesh dobby spot sleeves and decolletage, by Saint Laurent (an eye-popping £1,421!), with a pair of elegant black pointed court shoes.
The LBD, which finished just above the knee and featured a sweetheart neckline, showed off Naomi's perfect pins as she arrived at the venue.
While the dress looked fantastic, unfortunately it looked like Naomi had come face to face with the perils of applying make-up on the go, and had powdered her nose a little too liberally. Oops!
Never mind Naomi, we've all been there...