Bentley Brook is a stream located in Derbyshire, England. It rises on Matlock Moor, flowing south through Cuckoostone Dale, under the A632, into Lumsdale, gathering the valley's waters—notably from Knabhall Brook, out of Tansley, itself dammed and supporting large mills. In Lumsdale it enters a now disused mill pond or reservoir, then flows over a waterfall in the course of passing several historical mill ruins. Finally, it runs through control gates into the outside bend of a tight oxbow of the River Derwent, just beyond Hall Leys Park in Matlock.