Police say a convicted armed robber has robbed a bank for a second time after escaping from a Perth hospital.

Ronald Rowles, 51, escaped from custody shortly before 1:00pm on Wednesday.

He was about to be treated for a medical condition at Royal Perth Hospital when he gave prison guards the slip.

Police say Rowles later robbed a Westpac Bank branch in Northbridge, leaving with an undisclosed amount of money.

They say it was the same branch in James Street where he committed an armed robbery in June 2010.

On that occasion, Rowles handed over a withdrawal slip with demands written on it and left with a sum of money.

Police have cordoned off the area and are continuing their search.

Rowles is described as 170 centimetres tall, with a slim build and fair complexion.

He has grey hair and blue eyes, and was last seen wearing a green prison uniform.

Anyone who may have seen him is asked to contact police immediately.