Two men in balaclavas tied a staff member at a convenience store to a chair and set fire to the shop, Adelaide police said.
The victim managed to get free and raise the alarm.
Fire crews took about an hour to put out the blaze in the store in Jetty Road at Glenelg, about 3:30am ACST, having prevented its spread to apartments above the shop or neighbouring businesses.
The store worker was not injured but was taken to hospital as a precaution.
Police said the offenders were 175-178cm tall and wore dark clothes.
Fire cause investigators estimate the damage bill from the blaze at $250,000.