A man entered the Esso garage in Broomloan Road, Ibrox, at about 05:30 and threatened the male member of staff with a knife and demanded money.
He ran off with a three-figure sum of cash.
He was white and was wearing a blue jacket with a red scarf, blue jeans, white trainers and an orange baseball cap.
The member of staff was uninjured but shaken as a result of the robbery.
Det Sgt Peter Sharp said: "Inquiries are ongoing in the local area to trace the man responsible.
"I would urge anyone who saw anyone acting suspiciously near the garage early this morning to contact the robbery unit at Pollok Police Office through 101."
A petrol station attendant has been threatened with a knife during a robbery in Glasgow.