The garden, which is tended by Huntingdon’s disease patients in Wandsworth, received the accolade for its community planting activities and care research, which takes place at the site.
Chairman and judge of London in Bloom Geoff Hyde said: "It is an outstanding example of the benefits to a long-stay hospital community of patients, their visitors and staff that a garden of modest size can achieve. The careful and intensive management of the garden makes full use of its potential, for scientific research as well as pleasure."
A Royal Horticultural Society spokeswoman said the garden was a "wonderful sight". She added: "Flowerbeds are filled with shrubs, waist height accessible planters overflow with flowers and vegetables and pots are stuffed into every conceivable corner."
According to RHN, the garden offers patients an outdoor activity that promotes social interaction and provides therapeutic benefits including "improved mobility and reduced feelings of isolation and exclusion".
The other gardens to be recognised as outstanding at the awards were:
Alexandra Residents Association, Bromley
Bankside Open Spaces Trust, Southwark
Barn Hill Conservation Group, Brent
Biggin Hill Friends of the Park, Bromley
Blackstock Triangle Gardeners, Islington
BOST, Waterloo Millennium Green with Friends of Hatfields Green And Styles House Residents Association, Lambeth & Southwark
Culpeper Community Garden, Islington
Friends of Abbey Gardens, Newham
Friends of Archbishop’s Park Gardening Group, Lambeth
Friends of Eastcote House Gardens, Hillingdon
Friends of King Henrys Walk Garden, Islington
Goodman House Gardeners Group, Royal Hospital for Neuro-Disability, Wandsworth
Greenfingers & Friends of Archbishop’s Park, Lambeth
Hook Lane Primary School, Bexley
Long Lane Pasture Trust, Barnet
Phoenix School Farm and Learning Zone, Hammersmith & Fulham
The Friends of Barnes Common, Richmond
The Gardens’ Residents Association, Haringey
The Radiomarathon Centre, Enfield
Walworth Garden Farm, Southwark