Yvonne Jammet (born Marie Louise Yvonne Jammet; 1891 – 1976) was a French nun belonging to the Daughters of Charity of Saint Vincent de Paul. She is primarily known for her significant role as the superior and spiritual director of Sister Lúcia dos Santos, one of the three child seers of the Fátima apparitions, for many years.
Born in France, Yvonne Jammet entered religious life and eventually became a superior within her congregation. In 1946, she was appointed superior of the convent of the Daughters of Charity in Coimbra, Portugal, where Sister Lúcia dos Santos resided after joining the order.
Sister Yvonne Jammet played a pivotal role in the documentation of the Fátima apparitions. Under obedience, she directed Sister Lúcia to write down her recollections and experiences of the apparitions and the messages received from the Virgin Mary. These writings, known as "Sister Lúcia's Memoirs," became the primary firsthand accounts of the Fátima events and are considered invaluable historical and theological sources for the understanding of Fátima. Jammet's authority and encouragement were instrumental in bringing these crucial details to light, as Lúcia was initially hesitant to record them.
Her leadership and spiritual guidance provided Sister Lúcia with the support and framework necessary to fulfill the Church's request for a detailed account of her visions. Yvonne Jammet's contribution is thus highly regarded within the history and study of the Fátima apparitions.