The actress wore a 1960s-like dress with a pretty lace cut-out pattern across the chest. Watts, 44, posed for photos on the red carpet and signed autographs for fans.
She was joined at the event by co-star Naveen Andrews, who plays surgeon Dr. Hasnat Khan, and German director Oliver Hirschbiegel. Watts portrays Diana, Princess of Wales in the film, which centers on the late royal in the last years of her life before her death in a car crash in Paris in 1997.
"Diana" has received less than favorable reviews from critics in Britain, who have labeled it "fabulously awful," "tedious" and "cheap." The princess' former lover Dr. Khan insists the story is based on "hypotheses and gossip" and is "completely wrong."
The film hits U.S. theaters on Nov. 1.
Check out more photos of Naomi in Paris...