The Mar Saba letter is a Greek document which scholar Morton Smith reported in 1960 that he had discovered in the library of the Mar Saba monastery in 1958. The document has been lost and now only survives in two sets of photographs. The text purports to be an epistle of Clement of Alexandria and contains the only known references to a "Secret Gospel of Mark".

Morton Smith was an American professor of ancient history at Columbia University. He is best known for his reported discovery of the Mar Saba letter, a letter attributed to Clement of Alexandria containing excerpts from a Secret Gospel of Mark, during a visit to the monastery at Mar Saba in 1958. This letter fragment has had many names, from The Secret Gospel through The Mar Saba Fragment and the Theodoros.