Officers are urging drivers, including those that use their own vehicles to deliver parcels, to be extra vigilant while out completing their deliveries.
Police say thieves are stealing parcels and on occasion delivery vehicles, that could then be used to commit other crimes.
South Yorkshire Police said there has been an increase in delivery drivers being victims across the county.
Drivers are being urged to remember that car thieves and opportunists are in the area and they will take their chance to steal parcels and their vehicle.
Detective Inspector Ben Wood said: "We are seeing an increase in delivery drivers being targeted for theft.
"The reports we are receiving have a similar trend of drivers leaving their vehicles unlocked and thieves are taking advantage of this opportunity.
"Alongside this being distressing for the delivery driver and having an impact on our communities, stolen cars are more likely to be used in further crimes, which creates additional risks for our communities.
"We can all take simple steps to avoid this happening."
Officers are urging drivers to lock their car every time they get out of it, including as they deliver a parcel to a door.
Drivers are also warned not to advertise their personal belongings and keep mobile phones, money and satnavs out of view.
If a driver thinks someone is watching them they should report it to police on 101.