Transitus Fluvii (which means "passing through the river" in Latin), or Passage Du Fleuve (in French), also known as the witch's cipher, is an occult alphabet consisting of 22 characters described by Heinrich Cornelius Agrippa in his Third Book of Occult Philosophy (1533). It is a Hebriac alphabet, which means that it uses the same basic letters as Hebrew, and is similar to the related Malachim and Celestial alphabets. The name may refer to the crossing of the Euphrates river by the Jews on their return from Babylon to rebuild the Temple. This alphabet can also be found in Abraham de Balmis Peculium Abrae. Grammatica hebraea una cum latino, Venetiis, 1523, which precedes Agrippa's work by ten years. Also in Geoffroy Tory, Champ Fleury, Paris 1529, f. 76v ubi tamen: “Lettres Chaldaiques,” Giovanni Agostino Panteo's Voarchadumia contra alchimiam, Venice, 1530, pp. 545-46. V. Perrone Compagni, Cornelius Agrippa De occulta philosophia Libri tres, and Leiden: E.J. Brill, 1992, p. 491.

Joannes Pantheus claims that, while the Hebrew alphabet was entrusted to Moses and Enochian to Enoch, the Transitus Fluvii was entrusted to Abraham ("Voarchadumia contra alchimiam, ars distincta ab archimia et sophia, cum additionibus, proportinonibus numeris et figuris opportuni." Gallica – Bibliothèque nationale de France. 1550.) What the letters originally meant is unknown; however, scholars do know that it was written right to left, like Hebrew, and that it was not used for ordinary matters. It was used for ceremonial purposes, namely with the more powerful magics of witchcraft, such as spells, rites, and curses. Furthermore, just writing the script was said to be sufficient to put one in an altered state of consciousness, and make one more susceptible to signals from other dimensions.

Transitus Fluvii was one of the alphabets written on the walls of Rustin Parr's house.