The Legion of Mary, in Latin Legio Mariae, is a Catholic association of lay people who volunteer at the service of the Church. It was founded in Dublin, Ireland in 1921. It currently has over three million active members and over ten million auxiliary members (praying members).

To be in the Legion of Mary, one must be a practicing Catholic, faithful to the teaching of the Church. Active members serve God under the banner of Mary, doing spiritual work. The main activities of the Legion consist in meeting all men, poor or rich, young or old, as well as the marginalized of society (homeless, prostitutes, prisoners…), as well as non-Catholics.