The pair, one of whom has blond hair and a Nordic appearance, held a knife to the neck of a saleswoman at a shop on Frognerveien in Oslo and then ran away with valuables.
Shop staff told police that the second robber had darker skin and was wearing a black hooded top and a silver necklace.
On Sunday night, two men robbed a petrol station and a late-night convenience store, also at knife-point.
"It is too early to say whether there is a connection between this robbery and the two robberies that took place this weekend. But the two events are very similar," said Ola Krokan of the Oslo police.