Castle Hill is a suburb of Sydney, New South Wales, Australia, located 34 kilometres north-west of the Sydney central business district and 9.5 kilometres north of Parramatta. It is within the Hills District region, split between the local government areas of The Hills Shire and Hornsby Shire.

According to the 2016 census, there were 39,594 residents in Castle Hill. 57.8% of people were born in Australia. The most common countries of birth were China 6.8%, England 3.8%, India 3.1%, South Korea 2.3% and South Africa 1.8%. In Castle Hill 61.2% of people only spoke English at home. Other languages spoken at home included Mandarin 8.3%, Cantonese 4.2%, Korean 3.0%, Arabic 1.7% and Persian 1.6%. The most common responses for religion in Castle Hill were Catholic 26.8%, No Religion 23.5% and Anglican 14.6%.

The bulk of Castle Hill residents own private vehicles and travel to work using these, however bus services and the Sydney Metro Northwest run to the suburb.