St. Mary of the Immaculate Conception is a historic Roman Catholic church located at 600 E Sixth St. (6th and Pine Streets) in Wilmington, New Castle County, Delaware. St. Mary's is the only active church in Delaware founded by Saint John Neumann, Bishop of Philadelphia 1852–1860, who consecrated it on October 31, 1858. The church and adjacent St. Mary's school were the principal institutions for worship and the education and integration of thousands of Irish immigrants in Wilmington, most of whom lived in the parish upon first arriving. The church was added to the National Register of Historic Places in 1976.