Thalang Police were called to the scene, a 7-Eleven store in Baan Para, Moo 4, Pa Khlok, at 3am.
The man entered the store wearing blue jeans and a grey long-sleeved jacket, and a black full-face helmet obscuring his face, the employee told police.
The man, who stood about 165cm tall, threatened the staffer with a knife to hand over cash from the register. The staffer obliged.
The robber made off with about B4,200 in cash, the employee said.
The robber was last seen speeding off on a motorbike towards Baan Laem Sai, the employee confirmed.
The staffer was unable to identify the make and model of the motorbike due to the darkness, police reported.
Thalang Police Chief Col Pisit Chuenphet has tasked Thalang Police Deputy Chief of Investigation Lt Col Suchart Luecha with tracking down the robber.
Lt Col Suchart has experience in the area as he used to serve as Pa Khlok Subdistrict Patrol Chief, explained a Thalang Police report.
Police are now checking CCTV footage from the store and other cameras in the area to track down the robber and bring him to justice, Thalang Police said.