Robert Arthur "Rob" Knox (21 August 1989 – 24 May 2008) was an English actor who portrayed the character of Marcus Belby in the film Harry Potter and the Half-Blood Prince (2009), and had signed to appear in the planned film Harry Potter and the Deathly Hallows (2010).

Knox died after being stabbed outside of a bar in Sidcup, Southeast London, in May 2008, when he intervened in a fight to protect his brother. His murder attracted widespread attention in the British press, and his assailant, Karl Norman Bishop, was subsequently sentenced to life imprisonment, with a minimum 20-year non-parole period.