Black Ditches is an earthwork close to the village of Cavenham of Suffolk, and part of it is a biological Site of Special Scientific Interest (SSSI).

It is described by the Suffolk Historic Environment Record as having no direct dating evidence but "by analogy with other linear earthworks in the region it is usually assumed to be post Roman".

The northern section of ditch remaining is around 0.65 miles (1.05 km) in length running across the eastern edge of Cavenham Heath towards the River Lark.

The southern section of the ditch is 1.25 miles (2.01 km) long, extending southwards in some form across Risby's Poor Heath.

Herb species include the locally rare Heath Sedge Carex ericetorum and Bastard Toadflax Comandra umbellata.