Police said on Tuesday the naked male corpse was found by a couple in a car park in the southwestern town of Darmstadt some two miles from the castle, but added it had probably been moved.
The medieval castle was the birthplace in 1673 of Johann Konrad Dippel von Frankenstein, a controversial alchemist famous for his avid interest in the creation of artificial life. He was also rumored to have experimented with human body parts.
Many say he inspired the character of Victor Frankenstein in Mary Shelley's 1818 novel, itself later the source of countless spin-offs and horror films. Dippel died mysteriously in 1734.
Local police dismissed suggestions of any link between the castle, a popular tourist attraction among gothic thrill-seekers, and the decapitated man as "absurd."