In 48 hours, the national animal charity has been alerted to a number of distressed owners saying they have been contacted by someone claiming to be from the RSPCA and saying their missing pet has been found.
The fraudsters then ask for bank details over the phone, either to pay for life-saving surgery, vet's fees, or fees for the pets to be returned.
A spokesman for the charity said: "Members of the public have been contacted by someone claiming to be from the RSPCA. They've been told that their missing pet is with the charity or at a local vets and they must pay money over the phone.
"In some cases they are asking for a small collection fee and owners are then finding much more money has been taken from their account and in others they are asking £200 for life-saving surgery.
In some instances, the fraudsters are mirroring our phone number so the incoming call displays 0300 123 8585 which is particularly concerning.
We believe that these people are scouring websites and social media to find missing pet posters and are targeting owners.
"It's disgraceful that these scammers are preying on already distressed pet owners and we're incredibly concerned that they may end up pocketing hundreds of pounds from owners who are desperately looking for their missing pets."
The RSPCA do not ask for money for veterinary care for missing pets over the phone in this way, and would urge anyone who receives such a call not to give out their personal details.
Anyone who is contacted about a missing pet in our care should ask for a reference number and the officer's name and call our national call centre on 0300 1234 999 (between 7am and 10pm).
If the call is not genuine, they should contact the police on 101.