Ian Hunter, 59, was caught with 250 images of the young Harry Potter star, despite having already been banned from having any computer equipment after previously stalking girls and viewing child pornography online.
According to The Mirror, the images of Watson were not indecent but the judge still sentenced Hunter to eight months in prison.
After the defendant admitted breaching a sexual offences prevention order, Judge Roger Chapple warned him: "What makes this serious is your persistence."
Watson, 21, and her Harry Potter co-stars Daniel Radcliffe and Rupert Grint have now finished filming the series but she has said that Hermione Granger was a "lovely" role to play.
She revealed: "I feel like earlier on I played a parody of myself. She was kind of this big personality and she's developed into someone much more human."
Harry Potter and the Deathly Hallows: Part 2 opens in cinemas in the UK and US on July 15.
> Gallery: 'Harry Potter and the Deathly Hallows: Pt 2'
Watch the trailer for Harry Potter and the Deathly Hallows: Part 2 below: