Castlereagh is a suburb of Sydney, in the state of New South Wales, Australia. Castlereagh is 67 kilometres 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.
The suburb is one of the most historic sites in Australia's history, being one of the five Macquarie towns officially proclaimed on 6 December 1810. Governor Lachlan Macquarie recorded the following in his journal “the Township for the Evan or Nepean District I have named Castlereagh in honor (sic) of Lord Viscount Castlereagh”, Secretary of State for War and the Colonies 1805–1806. He, in turn, was named after Castlereagh in Ireland. The official Government and General Order issued from Government House, Sydney was dated 15 December 1810.
Castlereagh Post Office opened on 1 April 1857 and closed in 1967.
After two hundred years, the rich river flats still provide for a thriving agricultural industry along the Nepean River. Many of Penrith district's pioneering families had originated from Castlereagh. The suburb's historical importance is reflected in its many surviving farmhouses, outbuildings, churches and cemeteries.