Three men claiming to be from Anglian Water gained entry to a house in Clipston and stole jewellery and a handbag.
The incident happened at about 9pm on Wednesday, March 9, in Naseby Road when the occupant opened the door to a man who said he needed to check the water due to the floods.
He came into the house and asked her to sit and wait, at which point the elderly victim realised there were other men upstairs.
All of them left the house after a short while.
The offenders are described as white, and one is about 30, 5ft 10in with a medium build, clean shaven with rosy cheeks and short dark hair.
He wore a dark trousers, gloves and a hat.
Another is in his 40s with a moustache and wore a hat, a checked top, dark trousers and gloves.
Anyone with information or who saw anything can contact Northamptonshire police on 101 or they can call Crimestoppers anonymously on 0800 555111.
Anyone who has suspicions or concerns that distraction burglars or rogue doorstep traders are operating in their neighbourhood or targeting someone they know can call 101 or the dedicated doorstep crime number on 0345 2307702.
For Trading Standards advice call 0845 4040506.
A police spokesman said: “We are urging people to be vigilant and advise people never to open the door to anyone they are not expecting.”
For more crime prevention advice go to www.northants.police.uk/lookout.