The 22-year-old Harry Potter star wore a black Dior mini-dress with a sweetheart neckline over a pair of black Dior trousers, which she accessorised with a Tiffany twist heart key pendant.
Watson stars alongside Logan Lerman and Ezra Miller in the coming-of-age movie directed by Stephen Chbosky, who wrote the novel on which it is based.
Speaking about the film on Nick Grimshaw's BBC Radio 1 Breakfast Show, the actress said: "It's really funny because when I watch the film back, there are a few things I can't watch. I can't watch the Rocky Horror dancing scene, and I can't watch the kissing scenes."
The star, currently dating Boston-based fellow Brown university student Will Adamowicz, arrived at the central London screening on her own.
Flick through Emma Watson's changing style through the years below: