The object is "large, thick, made of leather filled with bristles, and has a wooden tip," the office said in a news release.
"It was certainly expensive. Cleaning revealed it was made of high quality leather," Marcin Tyminski of the Regional Office for the Protection of Monuments told Radio Gdansk.
"This is an extraordinary find for people researching the history of erotica," he said.
The faux phallus was taken away for "maintenance," the office said.
Archaeologists said they also discovered pieces of pottery and jewelry in the latrine. They said remains of wooden swords in the area indicate the site may have been a fencing school.