West Mercia Police have issued the warning following thefts in Shrewsbury and Oswestry.
They reported that over the last few weeks three particular makes of vehicles have been targeted; Toyotas, Hondas and Mitsubishis.
Police have encouraged owners of any of these makes to take extra caution while officers continue to make enquiries to find those responsible.
Detective Inspector John Weaver said: “In the last few months we have seen spikes in this type of theft. I’d like to reassure the community that we are working hard to prevent such crimes and will be increasing our patrols.
“There are steps motorists can take to try and reduce the chance of your vehicle being targeted by thieves. If you do notice any suspicious activity where individuals are pulling up next to cars and interfering with a vehicle we would ask you to get in touch with us.
"It can take a thief less than three minutes to remove a catalytic converter from its housing on the car exhaust pipe but there are a number of ways the public can protect their property."
Some tips police shared to protect vehicles include:
Keeping it in a garage if possible
If parked on a driveway, install motion activated lighting
Otherwise, park in a well-lit, populated area
Buy a lock that can be fitted to the converter.
Call 999 for crimes in progress, or for non-emergencies report them online at www.westmercia.police.uk/report