Writing via his Twitter account, the acclaimed writer said that the "most high-profile actor of 2010" will appear in the superhero film.
Millar did not reveal any details about the star in question. Early rumours linked Mad Men's Jon Hamm with the title role, though this was later dismissed as speculation.
Debuting earlier this year, Superior is a creator-owned title that tells the story of a teenage multiple sclerosis sufferer who transforms into a superhero after acquiring an alien power.
An official announcement on the casting is expected in January 2011.