Pherecydes of Syros (/fəˈrɛsɪˌdiːz/; Ancient Greek: Φερεκύδης ὁ Σύριος; fl. 6th century BCE) was an Ancient Greek mythographer and proto-philosopher from the island of Syros. Little is known about his life and death. Some ancient testimonies counted Pherecydes among the Seven Sages of Greece, although he is generally believed to have lived in the generation after them. Others claim he may have been a teacher of Pythagoras, a student of Pittacus, or a well-traveled autodidact who had studied secret Phoenician books.

Pherecydes wrote a book on cosmogony, known as the "Pentemychos" or "Heptamychos". He was considered the first writer to communicate philosophical ideas in prose as opposed to verse. However, other than a few short fragments preserved in quotations from other ancient philosophers and a long fragment discovered on an Egyptian papyrus, his work is lost. However, it survived into the Hellenistic period and a significant amount of its content can be conjectured indirectly through ancient testimonies. His cosmogony was derived from three divine principles: Zas (Life), Cthonie (Earth), and Chronos (Time). In the narrative, Chronos creates the Classical elements and other gods in cavities within the earth. Later, Zas defeats the dragon Ophion in a battle for supremacy and throws him in Oceanus. Zas marries Chthoniê, who then becomes the recognizable Earth (Gê) with forests and mountains. Chronos retires from the world as creator, and Zas succeeds him as ruler and assigns all beings their place.

Pherecydes' cosmogony forms a bridge between the mythological thought of Hesiod and pre-Socratic Greek philosophy; Aristotle considered him one of the earliest thinkers to abandon traditional mythology in order to arrive at a systematic explanation of the world, although Plutarch, as well as many other writers, still gave him the title of theologus, as opposed to the later physiologoi of the Ionian school. Later hellenistic doxographers also considered him as one of the first thinkers to introduce a doctrine of the transmigration of souls to the Ancient Greek religion, which influenced the metempsychosis of Pythagoreanism, and the theogonies of Orphism. Various legends and miracles were ascribed to him, many of which tie him to the development of Pythagoreanism or Orphism.

Although it is relatively certain the Pherecydes was a native of the island of Syros, and that he lived in the 6th century BCE, almost nothing else is known about his life. There is even some discrepancy in the ancient sources of his life as to when exactly he lived within the 6th century. The Suda places his date of birth during the reign of King Alyattes in Lydia (c. 605-560 BCE), which would place him as a contemporary of the Seven Sages of Greece, among whose number he was occasionally included. Alternatively, Apollodorus, places his floruit several decades later, in the 59th Olympiad (544–541 BCE), a generation later. Assuming that Pherecydes was born in this later generation, younger than the philosopher Thales (624-545 BC) and thus an older contemporary of Anaximander, he would also be approximately the correct age for the Pythagorean tradition in which he is regarded as a teacher of Pythagoras. Most of the other biographical information is probably fiction, and the ambiguity and contradictions in the surviving testimonies suggest that any reliable biographical data that may have existed was no longer available in the Hellenistic period. The identity of Pherecydes was also unclear in ancient times because there were two authors of that name who both wrote about mythology: Pherecydes of Syros and Pherecydes of Athens (fl. 5th century BC).

According to a forged letter attributed to Thales, Pherecydes never traveled but according to other sources he traveled throughout the Greek cultural area, to Delphi, the Peloponnese, Ephesus and Samos. According to Josephus and Byzantine writers, Pherecydes also made a journey to Egypt. Such a journey, however, is a common tale that is also part of other biographies of philosophers. A sun-dial (heliotropion), supposedly made by Pherecydes, was said by Diogenes Laërtius to be "preserved on the island of Syros." Several miraculous deeds were also attributed to Pherecydes; such as that he accurately predicted an earthquake on Syros after drinking from a well, or that he predicted the sinking of a ship that he saw along the coast of Syros, which then proceeded to sink. In Messene he allegedly warned his friend Perilaus that the city would be conquered. Finally, Hercules was said to have visited him in a dream and told him to tell the Spartans not to value silver or gold, and that same night Heracles is said to have told the king of Sparta in his sleep to listen to Pherecydes. Many of those miracles however, were also attributed to other legendary philosophers such as Pythagoras or Epimenides.

There are many conflicting legends that purport to be an account of the death of Pherecydes. According to one story, Pherecydes was killed and skinned as a sacrifice by the Spartans, and their king kept the skin out of respect for Pherecydes' wisdom. However, the same story was also told about Epimenides. Other accounts have the philosopher perishing in a battle between the Ephesians and Magnesians, or throwing himself from Mount Corycus in Delphi, or succumbing to typhoid fever. According to Aelianus typhoid fever was a punishment for his wickedness . The latter story was already known to Aristotle and may have arisen from the idea that wise men did not care about physical care. Other stories connect Pherecydes' death to Pythagoras. However, the historicity of all this is debatable.

Pherecydes was designated as 'wise' (sophos), but only Servius calls him a philosopher (philosophus). Aristotle places him between theologians and philosophers, because he no longer expressed himself completely mythical in his research. No consistent teacher of Pherecydes was known by name in late antiquity; according to the doxographer Diogenes Laërtius Pherecydes was taught by Pittacus, but according to the Suda he taught himself after he got his hands on 'the secret books of the Phoenicians'.

Although this latter claim is almost certainly fictitious, it may be based on the similarity between Pherecydes' ideas and Eastern religious motifs. For example, in his book he describes an important battle in the earliest times between Kronos and Ophion, and this motif occurs in the Middle East. His father was named Babys, a name that presumably originated from southern Anatolia, based on linguistic evidence. Eternal time as god is also Middle Eastern. In addition, Pherecydes has been associated with Zoroastrianism. Isidore the Gnostic claimed that Pherecydes based his allegorical work on a 'prophecy of Ham'. Ham, as referred to here, may be Zoroaster, who was quite well known in the Greek world of late antiquity. Isidore may have concluded this because the Zoroastrian literature available to him was influenced by Hellenization, or because Pherecydes' work influenced it. There is also a short fragment in which Pherecydes talks about ambrosia of the moon, the potion of the gods. This representation has parallels in the Samaveda, where there the moon is a vessel from which the gods drink soma (god drink) and is important in the reincarnation theory as guardian of heaven.