The actor who played young Tom Riddle in Harry Potter and the Chamber of Secrets was Christian Coulson.
Coulson portrayed the sixteen-year-old Tom Marvolo Riddle, whose preserved memory appears in the diary found by Ginny Weasley. Riddle initially presents himself as a charming former Hogwarts student, but Harry eventually learns that he is the teenage form of Lord Voldemort. The performance is central to the film’s mystery because Coulson’s calm, polite manner gradually gives way to Riddle’s manipulative and ruthless personality.
Coulson was 23 when he played the sixteen-year-old Riddle. He later did not return for the younger version of the character in Harry Potter and the Half-Blood Prince because he was considered too old for the role by then. That part went to Hero Fiennes-Tiffin, while Frank Dillane played the older teenage Riddle in the same film. Ralph Fiennes, meanwhile, played the adult Lord Voldemort.