Hermes Trismegistus (from Ancient Greek: Ἑρμῆς ὁ Τρισμέγιστος, "Hermes the Thrice-Greatest") is a legendary Hellenistic period figure that originated as a syncretic combination of the Greek god Hermes and the Egyptian god Thoth. He is the purported author of the Hermetica, a widely diverse series of ancient and medieval pseudepigraphica that laid the basis of various philosophical systems known as Hermeticism.

The wisdom attributed to this figure in antiquity combined a knowledge of both the material and the spiritual world, which rendered the writings attributed to him of great relevance to those who were interested in the interrelationship between the material and the divine.

The figure of Hermes Trismegistus can also be found in both Muslim and Baháʼí writings. In those traditions, Hermes Trismegistus has been associated with the prophet Idris (the Biblical Enoch).

Hermes Trismegistus may be associated with the Greek god Hermes and the Egyptian god Thoth. Greeks in the Ptolemaic Kingdom of Egypt recognized the equivalence of Hermes and Thoth through the interpretatio graeca. Consequently, the two gods were worshiped as one, in what had been the Temple of Thoth in Khemenu, which was known in the Hellenistic period as Hermopolis.

Hermes, the Greek god of interpretive communication, was combined with Thoth, the Egyptian god of wisdom. The Egyptian priest and polymath Imhotep had been deified long after his death, and therefore assimilated to Thoth in the classical and Hellenistic periods. The renowned scribe Amenhotep and a wise man named Teôs were coequal deities of wisdom, science, and medicine; and, thus, they were placed alongside Imhotep in shrines dedicated to Thoth–Hermes during the Ptolemaic Kingdom.

Cicero enumerates several deities referred to as "Hermes":

The most likely interpretation of this passage is as two variants on the same syncretism of Greek Hermes and Egyptian Thoth (or sometimes other gods): The fourth (where Hermes turns out "actually" to have been a "son of the Nile", (i.e. a native Egyptian god) is the Egyptian perspective, the fifth (who went from Greece to Egypt) is the Arcadian Greek perspective. Both of these early references in Cicero (the oldest Trismegistus material is from the early centuries CE) corroborate the view that Thrice-Great Hermes originated in Hellenistic Egypt through syncretism between Greek and Egyptian gods (the Hermetica refer most often to Thoth and Amun).

The Hermetic literature of the Egyptians was concerned with conjuring spirits and animating statues, the newly developed practice of alchemy, and informs the oldest Hellenistic writings on Greco-Babylonian astrology. In a parallel tradition, Hermetic philosophy rationalized and systematized religious cult practices and offered the adept a means of personal ascension from the constraints of physical being. This latter tradition has led to the conflation of Hermeticism with the contemporaneously developing, but distinct, Gnosticism.