According to reports, German movie studio Constantin Film suggested that Watson has been cast as college student Anastasia Steele.
But the 22-year-old took to her Twitter page to respond to the speculation, writing, "Who here actually thinks I would do 50 Shades of Grey as a movie? Like really. For real. In real life."
The star later added, "Good. Well that's sorted then."
Mila Kunis, Lily Collins and Nina Dobrev are among the others who have been tipped to play author EL James' literary heroine in the much-anticipated film, while Ian Somerhalder is a fan favourite to play wealthy entrepreneur Christian Grey.