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 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.

This article about a property in St. Charles County, Missouri on the National Register of Historic Places is a stub.