Police were called to Gledhow Valley Woods in north Leeds shortly after 11am when the weapon was found by a member of the group.
A spokesman said the sawn-off shotgun had been wrapped up in a piece of old carpet and would now undergo a full forensic examination.
Firearms trained officers were sent to the scene initially to make the shotgun safe to be moved.
An area of the woods was cordoned off while the officers worked and to allow forensic work to be carried out at the scene.
A full investigation will now be carried out by Leeds District CID detectives.