St. Mary's Cathedral is a Roman Catholic cathedral located in Sydney, Australia. It is the seat of the Archbishop of Sydney and serves as the mother church of the Archdiocese of Sydney. The cathedral is one of the largest and most ornate churches in Australia, and it is dedicated to the Blessed Virgin Mary, under the title of "Our Lady Help of Christians."

Construction on St. Mary's Cathedral began in 1868 and was completed in 1928. The building was designed by William Wardell, a noted architect who also designed other significant buildings in Australia. The cathedral features Gothic Revival architecture, with elaborate stained glass windows, soaring arches, and intricate carvings. The interior is adorned with beautiful mosaics and statues, and the cathedral's grand organ is one of the largest in Australia.

St. Mary's Cathedral has played an important role in the history of the Catholic Church in Australia. It was the site of the first Catholic Mass celebrated in Australia in 1803, and it has served as the venue for many important religious events, including the canonization of Saint Mary MacKillop in 2010. The cathedral has also been a place of refuge and support for many immigrants and disadvantaged groups throughout its history.

Today, St. Mary's Cathedral is a popular tourist destination and a place of worship for Catholics in Sydney. It is also a venue for concerts, cultural events, and other gatherings. Visitors can take guided tours of the cathedral to learn more about its history and architecture, and the cathedral also hosts regular Masses and other religious services.