Hermes Trismegistus, meaning "Thrice-Greatest Hermes," is a legendary authorial figure from Roman Egypt, most active in the 2nd and 3rd centuries CE. He is not a historical person but a pseudonym representing a fusion of the Greek god Hermes and the Egyptian god Thoth. This name was used by multiple writers within a multicultural tradition known as Hermeticism, which blended Greek philosophy with Egyptian religious and magical ideas.

The works attributed to Hermes Trismegistus cover philosophy, cosmology, and the technical arts, especially alchemy. The most famous collection is the Corpus Hermeticum, a series of Greek dialogues on theology and the nature of the soul. Other significant texts include the Asclepius and the cosmological dialogue Korē Kosmou. A large number of pseudepigraphic works on alchemy and astrology also bear his name, forming a vast and fragmented corpus.

According to modern scholars, Hermes Trismegistus is a central figure in the Western esoteric tradition. The Hermetic writings were influential in late antiquity and were later preserved in the Byzantine and Islamic worlds, where Hermes was revered as a prophet of science. The texts were rediscovered in Western Europe during the 15th century, sparking a Renaissance revival of "ancient theology." This revival profoundly influenced philosophers, scientists, and alchemists, cementing the name of Hermes Trismegistus as the legendary founder of alchemy and a symbol of ancient wisdom for centuries.