A woman in her 90s was approached by a man in the Ashton area of the city as she returned home from a shop at around 6.45pm on Sunday (May 10).
The man said he had been sent by the police with an update on a previous incident so she let him into her home.
She later found that her purse had been stolen.
An investigation is underway to find him and officers are warning others to be vigilant.
Insp Chris Barton, of Preston Police, said: "This is a despicable crime committed against an elderly and vulnerable lady and we will do all we can to track down this callous offender.
"I would urge people to be vigilant and to take some simple steps to try to avoid becoming a victim.
“A bogus caller may call uninvited to your home with the intention of tricking you into letting them in so that they can steal from you."
Police have warned rogue traders may advise you that you need work doing on your roof, driveway or other part of your home.
They may even say that your property is dangerous and that you will be responsible if someone is injured.
The work they do can be of poor standard and their prices are often inflated.
Insp Chris Barton added: "Older adults in particular are at risk but anyone can become a victim."
Police have released the following advice to help people protect themselves from rogue traders:
STOP - Before you answer the door, stop and think if you are expecting anyone. Make sure your back door and any other doors leading outside are locked before you go to the front door. If you feel uncomfortable don’t answer the door and lock it.
CHAIN - Open the door with the chain or door bar in place. These act as a barrier when talking to unexpected callers. Talk through a window if you can.
CHECK - Always ask for ID. Genuine callers will always have identification. If you are still uncertain, phone the organisation and confirm the person’s identity. Genuine callers will be happy to wait.
If you are not expecting a visitor, do not know who is at the door, have any doubt or feel uncomfortable, do not open the door.
Some companies also have password protection schemes where they supply you with a password only you and the caller will know.
Neighbourhood Watch schemes are also proven to reduce burglary and deter bogus callers, according to police.
If you think you have been a victim of fraud, report it to Action Fraud, a national agency who record and investigate reports of all types of fraud.
You can contact Action Fraud by calling 0300 123 2040 or via their website https://www.actionfraud.police.uk/For general advice about rogue traders, call Citizens Advice on 0345 404 0506.