St. Mary's Institute of O'Fallon, Historic convent and school in O'Fallon, United States

St. Mary's Institute of O'Fallon is a convent, school and motherhouse complex whose main building is a three-story Gothic Revival motherhouse. Its original section dates from 1874, and interconnected wings added through 1997 give the building an irregular plan. Inside are the convent, two chapels, an academy, a novitiate, dining room, kitchen, gym and infirmary; part of the building houses O'Fallon City Hall.

The motherhouse's original section was built in 1874, and interconnected wings were added in stages through 1997, giving the building its irregular plan. The complex was listed on the National Register of Historic Places in 2007 as a district of 11 contributing buildings and one contributing site, a cemetery.

The motherhouse belongs to the Congregation of the Sisters of the Adoration of the Most Precious Blood, whose name refers to adoration of the Eucharist. The complex holds two chapels: the larger Chapel of the Most Precious Blood, used for services and events, and the smaller Chapel of Our Lady of Sorrows, used by the sisters for daily prayer. The academy, opened in 1875, taught girls until it closed in 1970 and now serves as a museum on the school's history.

The historic district covers 11 contributing buildings and one contributing site, a cemetery. Part of the motherhouse now houses O'Fallon City Hall, so some sections are municipal offices rather than convent space. The complex sits in O'Fallon, St. Charles County, Missouri.

The historic district counts 11 contributing buildings plus one contributing site, a cemetery. The motherhouse grew by interconnected wings added between 1874 and 1997, giving the complex an irregular plan rather than a single construction date. Part of the building now houses O'Fallon City Hall.