The man, who staff described as fair skinned and about 185 cm tall, was wearing a block hoodie and had a scarf wrapped around his face.
"The employees were asked to lie down and the robber got away with an undisclosed amount of money," Victor Jensen from the local Rogaland Police told VG. "The perpetrator fled on foot. We have several patrols out looking for him."
According to the police, the man may did not appear to be stressed or intoxicated.
"During the robbery, he seemed cool and calm," Jensen said.
The police were first notified about the incident at 2.40am.