The incident took place yesterday (Monday) in Thelwall Lane, Latchford, at around noon.
A man knocked on the door of a property claiming to be from the water board. He claimed to be carrying out work further along the road and said he needed to gain access to the property to turn off the water.
The victim, an 86-year-old woman, let the man inside.
She was asked to go upstairs to the bathroom while the man went into the lounge, searched through a handbag and stole a quantity of cash. He then left on foot.
Police say the man was aged 40-50, around 5ft 5 inches tall, of stocky build and with short, dark greying hair.
He was wearing clothing similar to that of a workman, including a light brown sleeveless leather jacket and workman's boots.
DC Stuart Eales said: "We are urging anyone who witnessed the incident or has any information, which might assist with our enquiries, to get in touch.
"We want to reassure members of the local community that we are doing all we can to trace whoever is responsible."
Anyone with any information is urged to call Cheshire Police on 101.
Information can also be passed on anonymously, via Crimestoppers, on 0800 555 111.