"Creator ineffabilis" (Latin for "O Creator Ineffable") is a Christian prayer composed by the 13th-century Doctor of the Church Thomas Aquinas. It is also called the "Prayer of the St. Thomas Aquinas Before Study" (Latin: Orátio S. Thomæ Aquinátis ante stúdium) because St. Thomas "would often recite this prayer before he began his studies, writing, or preaching."[1] Pope Pius XI published this prayer in his 1923 encyclical letter on St. Thomas Aquinas, Studiórum Ducem.[2] It is now among the most well-known prayers attributed to Saint Thomas Aquinas.[3]