St Mary's Catholic Church, Williamstown, Gothic Revival Catholic church in Williamstown, Australia

St Mary's Church, officially the Church of St Mary of the Immaculate Conception, is a Roman Catholic church in Williamstown, Melbourne. The present stone building, designed by William Wardell, has a spire, transepts, sanctuary, side chapels and sacristy added in 1933 to the original design of W. P. Connolly.

The church was founded in the early 1800s and was the second Catholic church built in the Port Phillip District of New South Wales. Its original design was modified as resources were redirected to support the construction of St Patrick's Cathedral in Melbourne.

The parish keeps a register of baptisms that records the names of its priests and the dates of their service, a resource for tracing family and parish history. The church is open to visitors daily. Its pipe organ is documented by the Organ Historical Trust of Australia.

The church is located on Cecil Street in Williamstown and easy to find in the town centre. Visitors can attend regular services, and the building is connected to a nearby Catholic primary school.

William Wardell's original plans for St Mary's were lost at one stage, and the archdiocese then modified the design to free resources for St Patrick's Cathedral in Melbourne. The steeple was only added in 1933, together with the transepts, sanctuary, side chapels and sacristy, following W. P. Connolly's original design.