The Diocese of Sault Sainte Marie, Ontario (French: Diocèse de Sault-Sainte-Marie, Latin: Dioecesis Sanctae Mariae Ormensis) was decreed on September 16, 1904 and is formed by the southern portions of the districts of Thunder Bay, Algoma, Sudbury and Nipissing.

Their three missions were abandoned after a number of years, but Claude-Jean Allouez found converts still adhering to their faith in 1667.

In 1668 the mission of Sault Sainte Marie was founded by the Jesuits and used as a base for expeditions to adjacent areas.

Priests who appear in historical accounts of the time include Gabriel Druillettes, Louis André, Henri Nouvel, and Pierre Bailloquet.

Expansion after this point was rapid with hospitals and schools added to the parishes, churches and missions that marked this growth.