The actress, who plays Diana in the film, was pictured wearing a strapless grey dress at the event in London on Thursday.
She posed for pictures with Jacki Nielen, who plays the Duke of Cambridge in the film, and Freddie Spry, who plays the Duke of Sussex.
Spencer tells the story of a weekend in the early 1990s when Diana joined the royal family for Christmas.
Directed by Chilean filmmaker Pablo Larrain, it imagines what might have happened over those few days, when her marriage to the Prince of Wales has gone cold but she is still obliged to join the family festivities.
The film’s UK premiere took place at the Royal Festival Hall as part of the BFI London Film Festival ahead of its release in the UK and Ireland on November 5.
Others cast in the film, which is written by Peaky Blinders’ creator Steven Knight include Timothy Spall, Sally Hawkins and Sean Harris.