Aesop (/ˈiːsɒp/ EE-sop; Ancient Greek: Αἴσωπος, Aísōpos; c. 620–564 BCE; formerly rendered as Æsop) was a Greek fabulist and storyteller credited with a number of fables now collectively known as Aesop's Fables. Although his existence remains unclear and no writings by him survive, numerous tales credited to him were gathered across the centuries and in many languages in a storytelling tradition that continues to this day. Many of the tales associated with him are characterized by anthropomorphic animal characters.

Scattered details of Aesop's life can be found in ancient sources, including Aristotle, Herodotus, and Plutarch. An ancient literary work called The Aesop Romance tells an episodic, probably highly fictional version of his life, including the traditional description of him as a strikingly ugly slave (δοῦλος) who by his cleverness acquires freedom and becomes an adviser to kings and city-states. Older spellings of his name have included Esop(e) and Isope. Depictions of Aesop in popular culture over the last 2,500 years have included many works of art and his appearance as a character in numerous books, films, plays, and television programs.

The name of Aesop is as widely known as any that has come down from Graeco-Roman antiquity [yet] it is far from certain whether a historical Aesop ever existed ... in the latter part of the fifth century something like a coherent Aesop legend appears, and Samos seems to be its home.

— Martin Litchfield West

The earliest Greek sources, including Aristotle, indicate that Aesop was born around 620 BCE in the Greek colony of Mesembria. A number of later writers from the Roman imperial period (including Phaedrus, who adapted the fables into Latin) say that he was born in Phrygia. The 3rd-century poet Callimachus called him "Aesop of Sardis," and the later writer Maximus of Tyre called him "the sage of Lydia."

By Aristotle and Herodotus we are told that Aesop was a slave in Samos; that his slave masters were first a man named Xanthus, and then a man named Iadmon; that he must eventually have been freed, since he argued as an advocate for a wealthy Samian; and that he met his end in the city of Delphi. Plutarch tells us that Aesop came to Delphi on a diplomatic mission from King Croesus of Lydia, that he insulted the Delphians, that he was sentenced to death on a trumped-up charge of temple theft, and that he was thrown from a cliff (after which the Delphians suffered pestilence and famine). Before this fatal episode, Aesop met with Periander of Corinth, where Plutarch has him dining with the Seven Sages of Greece and sitting beside his friend Solon, whom he had met in Sardis. (Leslie Kurke suggests that Aesop was himself "a popular contender for inclusion" in the list of Seven Sages.)

In 1965, Ben Edwin Perry, an Aesop scholar and compiler of the Perry Index, concluded that, due to problems of chronological reconciliation dating the death of Aesop and the reign of Croesus, "everything in the ancient testimony about Aesop that pertains to his associations with either Croesus or with any of the so-called Seven Wise Men of Greece must be reckoned as literary fiction." Perry likewise dismissed accounts of Aesop's death in Delphi as mere fictional legends. However, later research has established that a possible diplomatic mission for Croesus and a visit to Periander "are consistent with the year of Aesop's death." Still problematic is the story by Phaedrus, which has Aesop, in Athens, relating the fable of the frogs who asked for a king, because Phaedrus has this happening during the reign of Peisistratos, which occurred decades after the presumed date of Aesop's death.

Along with the scattered references in the ancient sources regarding the life and death of Aesop, there is a highly fictional biography now commonly called The Aesop Romance (also known as the Vita or The Life of Aesop or The Book of Xanthus the Philosopher and Aesop His Slave), "an anonymous work of Greek popular literature composed around the second century of our era ... Like The Alexander Romance, The Aesop Romance became a folkbook, a work that belonged to no one, and the occasional writer felt free to modify as it might suit him." Multiple, sometimes contradictory, versions of this work exist. The earliest known version was probably composed in the 1st century CE, but the story may have circulated in different versions for centuries before it was committed to writing, and certain elements can be shown to originate in the 4th century BCE. Scholars long dismissed any historical or biographical validity in The Aesop Romance; widespread study of the work began only toward the end of the 20th century.