A female member of staff was held at knifepoint in a filling station forecourt in Dungannon on Monday evening.
The would-be robber demanded money from the woman as she locking up for the evening.
The assailant was between 16 and 20-years-of-age, according to police.
A PSNI spokesperson said: “At around 8pm a female was closing up when a male aged 16-20yrs, slight build, 5’6” in height, approached her as she was in her own vehicle.
“Armed with a knife he demanded money. Staff here never carry money so he left empty handed.
“Suspect subsequently ran off into the trees and the lines.
“Obviously the victim is very shocked by what happened.
“Where you about the area at the time and notice someone matching the description going to or coming from the direction of Tesco’s.”
Contact police on 101 with any information.