Thornlie is a large residential suburb located in Perth, the capital city of Western Australia. It is situated approximately 15 kilometres south-east of the city's central business district and falls within the City of Gosnells local government area.
The geography of the suburb is characterized by the Canning River, which flows through its northern side. Since the 1950s, Thornlie has developed in five distinct stages: north-east Thornlie (1950s–60s), south Thornlie (1970s–80s), Crestwood (1970s), Castle Glen (1980s), and Forest Lakes.