Roger Sherman Loomis (1887–1966) was an American scholar and one of the foremost authorities on medieval and Arthurian literature. Loomis is perhaps best known for showing the roots of Arthurian legend, in particular the Holy Grail, in native Celtic mythology.

The Holy Grail is a treasure that serves as an important motif in Arthurian literature. Various traditions describe the Holy Grail as a cup, dish, or stone with miraculous healing powers, sometimes providing eternal youth or sustenance in infinite abundance, often guarded in the custody of the Fisher King and located in the hidden Grail castle. By analogy, any elusive object or goal of great significance may be perceived as a "holy grail" by those seeking such.