A 65-year-old woman was robbed of £300 at a cashpoint in Leytonstone.
It happened in the High Road outside the Lloyds TSB bank at around 2pm on Monday.
Police say as the woman entered her pin a man pushed her to the side saying "this machine isn't working".
He then withdrew the cash and ran off in the direction of the Green Man roundabout.
A Met spokesman said: "The suspect is described as an Asian man approximately 30 to 35-years-old around 5ft 9inches, of slim build with black hair and was wearing a black hooded jacket.
"Police are appealing for anyone with information about this incident to contact Detective Constable Raymond Anderson from Waltham Forest CID on 020 3276 0944 or to remain anonymous call Crimestoppers on 0800 555 111."
DC Ray Anderson said: "This is a despicable crime towards a vulnerable victim and we would urge anyone who has any information that may help identify this suspect to contact police so we can catch the person responsible."