Between 10 and 10:30 a.m. a white male brandishing a dark-colored handgun demanded cash from the clerk of the station at 404 E. Washington St.
The clerk complied and the suspect fled on foot heading either south or east. He made off with an undisclosed amount of cash.
Greencastle Police Detective Randy Seipel said the clerk was OK following the incident, "just shook up."
GPD, the Putnam County Sheriff's Department and DePauw Security are in search of the suspect.
He is described as a white male between 5-feet-2-inches and 5-feet-6-inches in height with dark hair and a thin mustache.
At the time of the robbery, he was wearing blue jeans and a gray hooded sweatshirt.
Anyone with information regarding the investigation should contact the Greencastle Police Department at 653-2925.