"Today we filed the petition to establish paternity on Prince Frederic's behalf," attorney Christopher Fields said.
Von Anhalt has said he carried on a decade-long affair with the Playboy centerfold and reality TV star, meeting periodically in hotels in San Francisco, Los Angeles and New York.
He joined two other men claiming to be the father of Smith's baby, Dannielynn.
Von Anhalt, who claims to be 59, is the eighth husband of the oft-married Gabor, 90. The two have been married for more than 20 years.