The comics creator has filed a lawsuit against former employee Al Simmons, claiming that he violated the terms of his employment and "breached his duty of loyalty", according to The Hollywood Reporter.
Simmons has published a book called The Art of Being Spawn in which he explains how he became the basis for the superhero, whose name he shares.
"Al Simmons, who was flattered and eagerly gave his consent to McFarlane in 1993 for his name to be a part of Spawn, was not the inspiration for Spawn's central character and no one has ever confused the character with Defendant Al Simmons," states the lawsuit.
"Curiously, Defendant Al Simmons has, over the years, as Spawn enjoyed popularity, remarked on how his association with Plaintiffs has provided him with some name recognition or notoriety, where he had none before Spawn."
It goes on to accuse Simmons of trading on McFarlane's fame.
Spawn centres around former assassin Albert Francis 'Al' Simmons who returns to Earth after making a pact with the Devil.
McFarlane recently settled his longstanding court case with fellow creator Neil Gaiman.