Castelnau is a road in Barnes, in the London Borough of Richmond upon Thames, south west London, approximately 5.1 miles (8.2 km) west from Charing Cross on the south side of the Thames river. About 1.1 miles (1.8 km) long, it is the main road south from Hammersmith Bridge and forms part of the A306 main road. It was originally named Upper Bridge Road.
The name Castelnau is also used informally for Castelnau Estate (see below).
Castelnau means 'new castle' in the Occitan language.
Three different English pronunciations of the word "Castelnau" seem to be in current use, all differing only in the final vowel: "castle know" is more ancient, and resembles the original French vowel, "castle now" is perhaps used to match with Nassau Road in the area, and "castle gnaw" is favoured by more recent inhabitants.
Castelnau takes it name from Castelnau-le-Lez, near Montpellier in France: in 1691, the 10th Baron of Castelnau and St Croix, a Huguenot, fled France for England following persecution,and his son, Charles Boileau, settled in north Barnes and his descendants developed parts of the area.
Major Charles Lestock Boileau built Castelnau Villas (now 84-122 and 91-125 Castelnau) in 1842, followed by rows of cottages called Castelnau Row, Castelnau Place and Gothic Cottagess. After his death in 1889, Upper Bridge Road was renamed Castelnau.
In 1926, London County Council built a cottage-style estate of 640 houses, called Castelnau Estate, on the site of a market garden. In 1971 these passed to ownership of Richmond upon Thames Council. Many are now privately owned. Many of the roads in this estate are named after Deans of St. Paul's who had been Lords of the manor of Barnes between the 14th and 17th centuries: Everdon, Kilmington, Alderbury, Kentwode, Howsman and Stillingfleet.
Castelnau is noted for 20 pairs of exceptional classical villas which were built in 1842 by Major Boileau (see above).
Castelnau and the Underground 
In 1902, a committee of the House of Lords considered two schemes relating to the possible extension of the London Underground railway that would have involved new lines beginning at Castelnau. In the end, neither was built.
The other was the Charing Cross, Hammersmith & District Railway that would have run from Castelnau to the Strand, via Hammersmith, Kensington and Piccadilly. Permission for the latter was refused, though elements of it were realised in the Piccadilly line (1906). The LUER withdrew its proposal for the Castelnau branch in the course of negotiations aimed at rationalising a number of competing schemes. The Underground never reached Castelnau.
Notable local places 
This area of Barnes has a number of notable places, some of which are in Castelnau itself.
- The Castelnau, a public house and restaurant at the north end of Castelnau, on the corner of Lonsdale Road. It was originally called the Boileau Arms
- Harrods Furniture Depository built in 1894 on the banks of the Thames east of Castelnau, is a landmark in the Oxford and Cambridge Boat Race. It now forms part of Harrods Village, an upmarket development of apartments
- Holy Trinity Church Barnes, in Castelnau
- St. Osmund's Roman Catholic Church, Castelnau
- The Leg of Mutton Nature Reserve
- St Paul's School, west of Castelnau, between Lonsdale Road and the river.
- 72C, the former home (2002-2006) of Lord Froom and the Right Honourable Daniel Chick.
- Colet Court (preparatory school), next to St Paul's School
- The Swedish School, west of Castelnau on the river
- Lowther Primary School
- The Harrodian School
- London Wetland Centre, Barnes, near the south end of Castelnau
- The old Olympic Studios, Barnes, near the south end of Castelnau
- Holy Trinity Church, Barnes
- St Paul's School
- Lowther Primary School
- The Swedish School in London
- Castelnau Chorale