It happened on the 700 block of Long Lane in Upper Darby, Pennsylvania around midnight Saturday,. April 10.
SEPTA released an image of the suspect who they say tried to board the Route 108 bus without a mask.
Investigators say the man pulled a gun and threatened the driver after being told masks are required to board.
A $1,000 reward is being offered for information leading to an arrest.
SEPTA is urging anyone with information about this individual to immediately call SEPTA Transit Police at (215) 580-8111.