Jamie Waylett, who plays bully Vincent Crabbe in the Harry Potter films, has been arrested on suspicion of possessing cannabis, the BBC reported.
A Scotland Yard spokesman told BBC that the 19-year-old actor and a friend were arrested on Thursday after police in London stopped the car they were in.
"The car was searched and officers found eight bags of a substance believed to be cannabis," he said. A raid on Waylett's home uncovered 10 plants, also thought to be cannabis.
Both men have been bailed until July, pending further inquiries and tests on the substances, police told BBC.
Warner Brothers, which releases the Potter movies, declined to comment on the arrest.
Waylett has appeared in all five Potter films as the right-hand man to Draco Malfoy (Tom Felton). Harry Potter and the Half-Blood Prince is due for release on July 17.