The Legion of Mary (Latin: Legio Mariae, postnominal abbreviation L.O.M.) is an international association of members of the Catholic Church who serve it on a voluntary basis. It was founded in Dublin, as a Marian movement by the layman and civil servant Frank Duff.

Today, active and auxiliary (praying) members make up a total of over 10 million members worldwide, making it the largest apostolic organization of lay people in the Catholic Church.Membership is highest in South Korea, Philippines, Brazil, Argentina and the Democratic Republic of Congo, which each have between 250,000 and 500,000 members.Membership is open to those who belong to the Catholic Church and believe in its teaching. Its stated mission is for active members to serve God under the banner of Mary by the corporal and spiritual works of Mercy, as mentioned in Chapter 33 of the Legion of Mary Handbook. The main apostolate of the Legion is activities directed towards Catholics and non-Catholics encouraging them in their faith or inviting them to become Catholic. This is usually done by encouraging them in prayer, attending Mass and learning more about the Catholic faith. The members of the Legion are engaged primarily in the performance of spiritual works of mercy, rather than works of material aid.