Officers from South Yorkshire Police’s Off Road Team have been targeting HMP Doncaster in Marshgate in recent weeks.
A spokesman said: “We have been carrying out perimeter patrols around the prisons in Doncaster to deter throw in’s, where by drugs are being catapulted over the walls.
"If you see anyone acting suspiciously around a prison then please ring in.”
Officers also gave details of other recent activities carried out in the region.
“Silverwood, Thrybergh is being targeted at night for the deer that reside there. If you see folk on there at night up to no good then ring it in. The nature reserve is being wrecked.
“The viaduct near Sprotbrough also saw lots of bike riders and walkers spoken to. This is an isolated spot that rarely gets patrolled so we pay it attention should there be anyone in crisis,” the statement added.