St. Mary's Institute of O'Fallon

St. Mary's Institute of O'Fallon, also known as the Motherhouse for the Congregation of the Sisters of the Adoration of the Most Precious Blood, is a historic convent, school, and national historic district located at O'Fallon, St. Charles County, Missouri. The district encompasses 11 contributing buildings and 1 contributing site (a cemetery). The main building is the three-story, Gothic Revival style motherhouse. Its original section was built in 1874, with a series of interconnected wings dating from 1874 through 1997 making for a complex, irregular plan building. The building includes the convent, two chapels, academy, novitiate, dining room, kitchen, gym, and infirmary. A part of the building houses the O'Fallon City Hall.

It was added to the National Register of Historic Places in 2007.