The force said 2013 to 2017 models of the car had been the victim of thefts in October.
It believes the vehicles are being stolen for parts and the shells disposed of.
Essex Police is urging owners to take action and ensure cars are protected from thefts.
A spokesman said: “Essex has seen over 100 Ford Fiesta’s reported stolen in October, these are 2013-2017 models.
“Our Stolen Vehicle Intelligence Unit suspect these vehicles are being stolen for parts and the shells disposed of, they encourage owners to ensure vehicles are double locked when left unattended, and to consider OBD protection, full steering locks, immobilisers.
“They suggest this type of vehicle is not stolen via relay attack; therefore, faraday pouches/bags will not prevent the theft.
“Focusing our patrols, working with our Automatic Number Plate Recognition Team (ANPR), conducting investigations and working with intelligence information will be at the forefront.
“However, we do need the support of our communities and encourage owners of all vehicles to remain vigilant, adding extra security features where possible.
“If you have witnessed any incidents, or have any information which will support us, please go to: http://esxpol.uk/HfOwE “Alternatively, if you would like to make an anonymous report you can contact independent charity Crimestoppers, by visiting their website or by calling 0800 555 111.”
It comes as the final Ford Fiesta rolled off the production line in July.
Ford announced in October last year that it would be stopping the Fiesta in 2023.
The Ford Fiesta was introduced in 1976 at a time when there was a need for smaller, more efficient cars, and became an instant hit. It is the most popular car sold in the UK ever and was the best-selling vehicle for 12 consecutive years up until 2021.
More than 22 million have been produced globally.
Essex Police said in Rochford District 18 vehicles were stolen, with Thurrock, Basildon and Southend also affected.