"There was a website set up that was threatening to release naked images of me, with like a countdown and whatever else. I knew it was a hoax. I knew the pictures didn't exist." she said during a live Facebook Q & A about gender equality.
"A lot of people close to me knew gender equality was an issue but didn't think it was that urgent. You know, 'We live in Great Britain - this is a thing of the past. We're fine. We're good. We have got far enough.'
The actress said the threat made her angry. "I was raging, it made me so angry that I was like, 'This is why I have to be doing this'... If they were trying to put me off it, it did the opposite.
"It's funny because people were like, 'She's going to be disheartened by this'. If anything, it made me so much more determined," she said.
Watson, who is a UN Goodwill Ambassador for Women, gave the interview to promote her campaign for gender equality HeForShe, which encourages men to stand up to address discrimination against women and girls.
"She went 'I just want them to want to play with us'... And I was like, well that translates at every level. We just want to be included," Watson said.
She also said that women were 'complicit' in bringing their own gender down.
"A lot of the criticism I've ever had in my life, some of the harshest moments of criticism or some of the hardest moments I've had, have been comments from other women.