A balaclava-clad man confronted staff and forced them to open the safe during a robbery at Betfred in Rothwell.
The robber escaped with cash from the bookies in Bridge Street, Rothwell, following the robbery on Tuesday (February 5).
A police spokesman said: “Sometime between 9pm and 9.20pm, a man walked into the shop and confronted staff, forced them to lock the door and access the safe from where he stole cash.
“The offender then smashed through the glass at the front of the shop to leave the premises after staff tried to lock him inside.”
Shopkeepers in the town said they were shocked by the incident. Claire Owens, from Trudi Claire – around the corner from Betfred – said the town needed more CCTV to stop people from carrying out similar offences.
And Frank Sindon, from the Bridge Street butchers, said recent robberies were worrying for shopkeepers.
He added: “I don’t know why it’s so bad here. It’s just idiots who do it.”
The offender was a white man, in his late 30s, about 6ft and well built.
He had blue/grey eyes, fair coloured hair and wore blue jeans, a dark blue hooded top, socks on his hands and a black balaclava.
Witnesses to this incident, or anyone with information about it, can call Northamptonshire Police on 101 or information can be given anonymously to Crimestoppers on 0800 555111.