John Blackwell is a character appearing in the CW television series The Secret Circle. He is the father of series protagonists Cassie Blake and Diana Meade, and serves as the main antagonist of the show's first and only season.

He is played by Joe Lando.

John returns to Chance Harbor and re-connects with his daughter Cassie after having been absent from her life. He then becomes an ally and mentor of sorts to Cassie and her friends, but it's revealed by the end of the season that he has a sinister agenda: eliminate all witches except those descended from the warlock Francis Balcoin (John's ancestor). After assembling a magical object called the Crystal Skull, he starts to set his genocidal plan into motion, but is stopped by his two daughters and Adam Conant, and is killed.

It soon turns out that John fathered other children apart from Cassie and Diana, and the show ends with a shot of said children standing together.