Mary Helen MacKillop RSJ, religious name Mary of the Cross, was an Australian religious sister of Scottish descent who has been declared a saint by the Catholic Church. She was born in Melbourne but is best known for her activities in South Australia. Together with Julian Tenison-Woods, she founded the Sisters of St Joseph of the Sacred Heart, a congregation of religious sisters that established a number of schools and welfare institutions throughout Australia and New Zealand, with an emphasis on education for the rural poor.

The Sisters of St Joseph of the Sacred Heart, often called the Josephites or Brown Joeys, are a Catholic religious order founded by Saint Mary MacKillop (1842–1909). Members of the congregation use the postnominal initials RSJ.