Cassiobury is a residential area of Watford in Hertfordshire, England.
It is formed by two areas of housing: one to the north-east of Cassiobury Park and the other to the south. Both are built on land that was formerly part of the estate of Cassiobury House, house of the Earls of Essex, which was located where Temple Close now sits.
It is favoured by commuters as it contains both Watford tube station with direct links to the City of London and the extensive Cassiobury Park. It is one of Watford's most expensive residential areas and houses a significant amount of Watford's upper middle class. It is very popular among young families as it gives direct access to Watford's most successful primary school, Cassiobury Junior School, from which a lot of students go on to either Watford Grammar School for Boys or Watford Grammar School for Girls, two of the country's top secondary schools. At the north end of Cassiobury, outside the Borough of Watford in the District of Three Rivers, is the former estate and house of the Earls of Clarendon. This has been converted into a luxury hotel and with extensive grounds called The Grove.