Retired police chief John Comparetto was attending the meeting of 300 officers when he was allegedly held up at gunpoint in the men's toilets.
He handed over money and a phone but then he and some colleagues gave chase as the suspect tried to flee in a taxi.
They arrested a 19-year-old man over the incident near Harrisburg.
Mr Comparetto was wearing an ankle holster with a gun, and when told to drop his trousers, he managed to conceal his weapon.
He described the suspect as "probably the dumbest criminal in Pennsylvania".
The Associated Press news agency reported that when a journalist asked the suspect for comment as he was led from court, he said: "I'm smooth."