Lonsdale Road Reservoir or Leg of Mutton Reservoir or Leg o' Mutton Reservoir is a disused reservoir in Barnes in the London Borough of Richmond upon Thames. It is an 8.2 hectare Local Nature Reserve and Site of Borough Importance for Nature Conservation, Grade 1, owned and managed by Richmond upon Thames London Borough Council.
Breeding birds include pochards, which are nationally scarce, mute swans, great crested grebe and grey herons. There are many more waterfowl in the winter. The site is also important for bats, and amphibians include the protected great crested newt. Aquatic plants include bogbean and frogbit, both of which are rare in London.
There is a mile-long perimeter path round the reservoir, which can be accessed from the Thames Path, Ferry Lane and Lonsdale Road.