The 29-year-old 'Begin Again' star has signed autographs pretending to be Spears, reported Contactmusic.
"It has happened to me three times in LA. I was quite excited by that and definitely signed and had a picture," she said.
The British beauty said she has very little in common with the singer, 32, as she hates singing in public.
"I like singing if I'm in my own in the shower or if I am really drunk, but I'm not a big fan of singing in front of people, which I didn't realise until I said yes to the movie and found myself singing in front of rather a lot of people," Knightley said.
The actress said she is also often mistaken for other celebrities, including Kate Winslet and Natalie Portman, in Los Angeles.