Harrow Weald Common is an 18-hectare area of woodland, heath and pasture in Harrow Weald in the London Borough of Harrow. It is considered of considerable importance for wildlife, and it was formerly part of the Stanmore and Harrow Weald Commons and Bentley Priory Site of Special Scientific Interest, but in 1987 the boundaries of the SSSI were revised to exclude the Common. It has been designated by the Mayor of London as a Site of Metropolitan Importance for Nature Conservation.
The site includes Grims' Dyke Open Space. Grim's Dyke or Grim's Ditch is an ancient earthwork which runs for three miles between Harrow Weald Common and Pinner Green. Its purpose is unknown, and it may date from the fifth or sixth centuries. Adjacent to the site are the City Open Space, Harrow Weald SSSI, a geological Site of Special Scientific Interest, and Bentley Priory Nature Reserve, a biological SSSI. There is access from Common Road and Old Redding.