Castlereagh is a suburb of Sydney, in the state of New South Wales, Australia. Castlereagh is 67 kilometres (42 mi) north-west of the Sydney central business district, in the local government area of the City of Penrith and is part of the Greater Western Sydney region.

According to the 2016 census, there were 1,171 residents in Castlereagh. 78.8% of people were born in Australia and 82.6% of people only spoke English at home. The most common responses for religion were Catholic 39.4%, Anglican 21.3% and No Religion 13.7%.

Castlereagh Road is one of the important transport links in Sydney connecting the Hawkesbury and Penrith regions.