Police have appealed for information to help find a thief who robbed an elderly woman while she was doing her banking in the Perth suburb of Belmont.

The victim, aged 90, was standing inside a bank at Belmont Forum Shopping Centre about 1:45pm on Wednesday when another woman approached her from behind.

The suspect reached into the victim's handbag and pulled out her wallet, which contained an unknown quantity of cash, before walking out of the bank with the wallet.

Police described the suspect as being dark-skinned, aged approximately 30, with a slim build and long dark hair.

She was wearing a white Stussy top, blue skinny jeans, white sneakers and dark sunglasses.

Police urged anyone with information in relation to the incident to call Crime Stoppers on 1800 333 000.