Lawyers for the 60-year-old star of the "Rocky" and "Rambo" movies entered the guilty plea on behalf of the actor, who did not appear before Sydney's Downing Center Local Court.
Stallone was accused of bringing a banned substance into Australia after a customs search of his luggage during a Feb. 16 visit to Sydney revealed 48 vials of the human growth hormone product, Jintropin.
Human growth hormone, a naturally occurring substance that can be replicated synthetically and is used to build muscle mass, is considered a performance enhancing drug in Australia and it cannot be imported without a permit.
Stallone faces a maximum penalty of a $22,000 fine. He was expected to be sentenced later Tuesday, and his lawyers made no immediately comment.
Prosecutor David Agius told Deputy Chief Magistrate Paul Cloran that Stallone was to have appeared in the court through a video link but that that plan had been abandoned. Agius did not elaborate.