It happened at about 5.30pm yesterday at the Spar in Catherington Lane, Horndean.
Two people walked in and approached a staff member at the till. One was holding a knife and demanded money.
A police spokeswoman said: ‘A substantial amount of money was handed over and the offenders left.’
They ran off towards the A3 and Morrisons.
The pair were white, between 15 and 18 and slim. One wore a black tracksuit with a distinctive emblem on the back, and the other wore a grey hoody and black tracksuit bottoms.
Police want to hear from anyone who was in the area at the time who saw anyone matching the descriptions, or anyone with any information.
The spokeswoman added: ‘Do you know anyone who has recently acquired a large quantity of cash with no explanation of where it’s from?’
Call 101 quoting 44220253469.