Cassiobury House was a country house in Cassiobury Park, Watford, England. It was the ancestral seat of the Earls of Essex. Originally a Tudor building, dating from 1546 for Sir Richard Morrison, it was substantially remodelled in the 17th and 19th centuries and ultimately demolished in 1927. The surrounding Cassiobury Park was turned into the main public open space for Watford.

Cassiobury Park is the principal public park in Watford, Hertfordshire, in England. It was created in 1909 from the purchase by Watford Borough Council of part of the estate of the Earls of Essex around Cassiobury House which was subsequently demolished in 1927. It comprises over 190 acres (77 ha) and extends from the A412 Rickmansworth Road in the east to the Grand Union Canal in the west, and lies to the south of the Watford suburb of Cassiobury, which was also created from the estate. The western part is a 62-acre (25.1 ha) Local Nature Reserve managed by the Herts and Middlesex Wildlife Trust. The park hosts the free, weekly timed parkrun 5 km event every Saturday morning at 9 am, starting on the field near the Shepherds Road entrance to the park, and finishing by the bandstand.