Cassiobury House was a country house in Watford, Hertfordshire, England, now demolished.
The house was started in 1546 by Sir Richard Morrison. On the marriage of his granddaughter it passed into the ownership of the Capel family, later Earls of Essex. It was demolished in 1927. Carvings from the staircase are now in the Metropolitan Museum of Art. The stables remain and were converted into a retirement home. Part of the grounds, but not the site of the house, now form the public Cassiobury Park.
The dower house of the estate "Little Cassiobury" still exists. Having been designed by Lady Elizabeth Wilbraham (the first woman architect), and built in 1670 it is listed as "grade II*" and "at risk" by English Heritage. Under the supervision of the Portmeirion architect Clough Williams-Ellis here were renovations and extensions to the house in 1937-38. Soon after that Hertfordshire County Council bought Little Cassiobury in 1939 under a compulsory purchase order. They used part of its land to build Watford College, and they continued to use the house as an education office for most of the 20th century.