St. Mary's Cathedral is a cathedral in Limerick, Ireland, dedicated to Mary.

In 1111 at the Synod of Rathbrassil, it was decided that "St. Mary's church" would become the cathedral church of the Diocese of Limerick . The present site of St. Mary's Cathedral may not have been the original location of the church of St. Mary. As Brian Hodkinson notes, tradition has it that Donal O'Brien, the King of Thomond, founded the present Cathedral on the site of his palace on King's Island in the latter half of the 12th century.

Since the Reformation, St Mary's has been part of the Church of Ireland. St. John's Cathedral, Limerick is the Roman Catholic cathedral.