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 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 25.1 hectare Local Nature Reserve managed by the Herts and Middlesex Wildlife Trust. The park hosts the free, weekly timed parkrun 5km event every Saturday morning at 9am, starting at the Rickmansworth Road entrance to the park.