An opportunist thief grabbed bank notes from a cash machine after the money was momentarily left dispensed.
A 49-year-old woman had withdrawn the money at Asda supermarket on Althorp Road in Hull and mistakenly left the notes in the machine as she walked away.
Moments later a man wearing a baseball cap and dark bottoms who was standing nearby took the money and did not return it to the woman.
The theft occurred at around 1pm on April 7.
Humberside Police are looking for the man and have asked anyone who may have seen him take the money to call them on 101 quoting 16/39938/18.