The robbery took place at Oldfields in Regent Street at 9pm when a member of staff was confronted by a man armed with a knife in the rear yard.
They went back inside where the man demanded cash and then left on a mountain bike with the takings. He was last seen on London Street.
The offender was white and aged 16 to 18. He was 5f 9in and slim. He was wearing a dark hooded top which had horizontal blue/grey stripes across the body, black tracksuit bottoms with white stripes on the outside of the legs, black trainers and a baseball cap with a white peak.
An Avon & Somerset police spokesman said: “We’d like to speak to anyone who recognises the description of the man or saw someone acting suspiciously in the area at that time.
“If you can help please contact us online or call 101 and quote 56649/14.”