Two men ordered staff at a town centre travel agency to go into the back office then took keys to the store and locked the front door before stealing a considerable amount of cash and travellers cheques.
Thomson in Silver Street, Wellingborough, was shut on Monday after the incident, which happened between 9am and 9.25am.
A Northamptonshire Police spokesman said two black men went in and told staff to go into the back office.
The spokesman said: “The offenders demanded the keys to the shop and then locked the front door. They then stole cash and travellers cheques before leaving the shop through the front door and turning left towards Morrisons and Oxford Street.
“Throughout the incident the offenders made it clear to the staff that they would not hurt them if they complied with their requests and reassured them when they became upset.”
One of the men wore a black coat and a scarf covering his mouth and nose and the other was dressed in black with a hood pulled up.
Police have been going through CCTV footage from the area and forensic officers visited the store on Monday.
A sign posted in the window of the store on Monday read: “Attention – Unfortunately there has been an incident and this shop will be closed for the remainder of the day.”
In a statement Thomson said: “Thomson can confirm that it is assisting police with their enquiries following a robbery to our Wellingborough shop on the morning of Monday, February 6.
“The regional sales manager and her deputy have already been on hand to offer assistance to those staff involved and the company will continue to support them in every way possible.
“Thomson is fully complying with the authorities and hope that the investigation comes to a swift and satisfactory conclusion.
“However, as the incident is subject to an ongoing police investigation we cannot comment further.”