In the latest incident, a man escaped with a three-figure sum after threatening two members of staff with a hammer at McColl's in West Doura Court, Kilwinning, at about 18:20 on Saturday.
Police said the staff were uninjured but "extremely distressed".
In December, a man robbed the same shop at knifepoint while wearing a 'Joker' mask.
The suspect in the latest robbery was described as being white, 5ft 10in, with a stocky build. He was wearing dark trousers and a black hooded top featuring the "McKenzie" logo.
Det Sgt Craig Semple said: "I'd like to speak to anyone who may have seen the suspect hanging around the area prior to the robbery taking place or anyone who saw him running out of the shop afterwards."
A convenience store in North Ayrshire has been robbed for the second time in five months.