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 (0.77 km2) 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.
