A web page titled Emma You Are Next, featuring an image of the Harry Potter star next to a countdown, suggests the actress will be targeted.
The page also features the logo of 4chan, the site which is said to have hosted two recent leaks of celebrity photos.
Jennifer Lawrence and Kim Kardashian are among those to be targeted.
The threat was launched hours after Emma Watson, 24, launched the HeForShe campaign in a speech at the UN in which she appealed to men to speak out over gender equality.
“I was appointed six months ago and the more I have spoken about feminism the more I have realized that fighting for women’s rights has too often become synonymous with man-hating,” Emma Watson said.
“If there is one thing I know for certain, it is that this has to stop.”
Emma Watson’s first appearance as Hermione Granger was in 2001’s Harry Potter and the Philosopher’s Stone as an 11-year-old.
She told UN delegates in New York that she started thinking about gender inequality as a child three years before hitting the big time.
UN Women, a United Nations group looking at gender equality, also suggested that the global film industry doesn’t do enough to represent women in films.
It said there had not been enough speaking roles for women and women have been under-represented in action and adventure movies.
It found 23% of speaking characters in that genre were female.
The study looked at films made in 11 countries, including South Korea, the US and India.
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