St Mary's Cathedral is the cathedral church of the Roman Catholic Archdiocese of Perth and seat of its Archbishop, currently Timothy Costelloe.

Officially the Cathedral of the Immaculate Conception of the Blessed Virgin Mary, the cathedral is located at the peak of a hill in East Perth, at the centre of Victoria Square in Western Australia.

The cathedral as it now stands was constructed in three main phases, with the first phase completed in 1865.

Plans were drawn up for the replacement of the cathedral in the 1920s with a larger Perpendicular Gothic edifice. However, construction was interrupted by the onset of the Great Depression, leaving a new transept and sanctuary, with the aisle of the original cathedral as its nave.

After 70 years being incomplete and portions of the cathedral requiring extensive repair work, funds were eventually raised in the late 1990s and early 2000s for the completion of the expansion. A new curved design for the expansion was created and featured a second spire and underground parish centre. The cathedral closed for construction in 2006 and reopened in December 2009.

It is open for prayer and quiet reflection every day.