The Legion of Mary (Latin: Legio Mariae) is an international association of the faithful of the Catholic Church who serve the Church on a voluntary basis. It was founded in Dublin, Ireland, as a Roman Catholic Marian Movement by layman Frank Duff. Today between active and auxiliary (praying) members there are over 10 million members worldwide making it the largest apostolic organization of lay people in the Catholic Church.

To be in the Legion of Mary, one must be a practicing Catholic. Active members serve God under the banner of Mary by practicing the Spiritual Works of Mercy.

The main apostolate of the Legion consists in activities directed towards all men and women, young and old, rich and poor as well as people from the margins of society (homeless, prostitutes, prisoners etc.) and towards non-Catholics. The members of the Legion are primarily engaged in the performance of the Spiritual Works of Mercy rather than works of material aid.