St. Maria Faustina Kowalska (1905–1938) of the Blessed Sacrament, was a Polish Roman Catholic nun and mystic. Her apparitions of Jesus Christ inspired the Roman Catholic devotion to the Divine Mercy and earned her the title of "Secretary of Divine Mercy".

All through her life, Kowalska reported having visions of Jesus and conversations with him, which she wrote in her diary, published as The Diary of Saint Maria Faustina Kowalska: Divine Mercy in My Soul. Her biography, submitted to the Congregation for the Causes of Saints, quoted some of these conversations with Jesus regarding the Divine Mercy devotion.

At the age of 20 years, she joined a convent in Warsaw, was transferred to Płock, and was later moved to Vilnius where she met her confessor Fr. Michał Sopoćko, who supported her devotion to the Divine Mercy. Kowalska and Sopoćko directed an artist to paint the first Divine Mercy image, based on Kowalska's vision of Jesus. Sopoćko used the image in celebrating the first Mass on the first Sunday after Easter. Subsequently, Pope John Paul II established the Feast of Divine Mercy on that Sunday of each liturgical year.

Kowalska was canonized as a saint on 30 April 2000. The mystic is classified in the liturgy as a virgin and is venerated within the church as the "Apostle of Divine Mercy". Her tomb is in Divine Mercy Sanctuary, Kraków-Łagiewniki, where she spent the end of her life and met confessor Józef Andrasz who also supported the message of mercy.