The 29-year-old dazzled in a long-sleeved gold Valentino dress with a tiered skirt and floral embroidered detail, from the designer's Haute Couture Autumn 2014 collection.
Keira Knightley at the premiere of new film The Imitation Game
Keira evoked old Hollywood glamour as she wore her glossy brown hair in loose waves with a side parting, and kept her makeup to a minimum.
The Atonement actress kept the focus on her dress with minimal accessories, matching her shoes to the red carpet, and opting for some sparkling statement earrings for a hint of extra glamour.
Earlier in the day, Keira and Benedict were spotted at a photocall ahead of the premiere as they spoke about the film, which sees the Sherlock actor starring as code-breaker Alan Turin.
Keira and Benedict Cumberbatch at the Corinthia Hotel in London
Keira showed off her fashion-forward style credentials in a red patterned dress from Valentino's Spring 2015 collection, which made its runway debut only a few weeks ago at Paris Fashion Week in September.
While the dress was paired with some flat black strappy sandals on the runway, Keira chose to team her gown with some black and gold heels, as she posed with Benedict at the Corinthia Hotel in London.