A cashier has been threatened with a knife during a supermarket holdup in Sydney’s inner west.

Police say a man thought to be in his mid-20s entered the Pyrmont supermarket armed with the weapon about 9.15pm (AEDT) on Monday.

He threatened the 21-year-old cashier and demanded cash from the register before fleeing with the takings.

Officers say the thief was Caucasian, about 180cm tall, of medium build and blue eyes.

AAP