St Joseph police seized a black Beretta .380 pistol with a magazine and one round of .38 ammunition on Monday, during a roving street-check exercise in the district.
The items were found on a 29-year-old man known to police.
A release on Tuesday said after a tipoff, police stopped the man on Farm Road, St Joseph near WASA's head office. He was in a grey Nissan Versa.
They found the ammunition in his wallet and the gun tucked in his waist.
He was arrested and taken to the St Joseph Police Station. Investigations are continuing.