The 22-year-old who played Hogwarts bully Vincent Crabbe in six of the films, was found guilty of violent disorder by a jury at London's Wood Green Crown Court.
The actor, who had already admitted swigging from a stolen bottle of Champagne during the rioting, was cleared of intending to destroy or damage property with a petrol bomb he was pictured holding.
He already has a previous conviction for cannabis possession.
Waylett of Hillfield Road, north-west London, was with a gang of at least four people who went into Chalk Farm on August 8, the third night of violence in the capital.
He was captured on CCTV at various points during the evening, often with his hood over his head. Read More