Perth Underground railway station is a railway station within the Perth central business district in Western Australia. It is adjacent to the above-ground Perth railway station and is sometimes considered part of that station. Perth Underground station is served by Joondalup line services heading north and Mandurah line services heading south. It was built as part of the construction for the Mandurah line and was known as William Street station during construction due to its location on William Street. Perth Underground station consists of an island platform and a concourse below ground. There are five entrances to the station: from Murray Street Mall, Raine Square, 140 William Street, underneath the Horseshoe Bridge, and from Perth station.

The Joondalup line is a commuter rail service in Western Australia, linking the Perth central business district (CBD) with the metropolitan area's north-western suburbs. The service is operated on the Northern Suburbs Railway by Transperth Train Operations, a division of the Public Transport Authority, and is part of the Transperth network. It is 40.7 kilometres (25.3 mi) long and serves 13 stations. The service is currently being extended to Yanchep.