Director Alfonso Cuaron said: "I am very excited to work with Emma. She brings such life and spirit to the characters she portrays."
There are reports that Azkaban will be a lot darker than the other films.
And, excitingly, pre-production has already started on fourth film, Goblet of Fire.
Thompson has appeared in lots of respected films, and joins other Brit actors Gary Oldman and David Thewlis as new additions to the Potter cast.
The film should be out in June 2004, and Cuaron has said it should feel a lot more up-to-date than the first two films.
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