Mid-Holderness PCSO Darren Bainton posted an image of the ordnance on Twitter after it was uncovered during ditch clearing.
It was discovered on Thirtleby Lane in the village of Coniston.
Police have now closed the road while an RAF bomb disposal team attends the scene. It is not known if the missile is live.
It has been speculated that the explosive dates from World War Two. Beaches in Holderness have been used as MOD testing sites in the past.