Erasistratus (/ˌɛrəˈsɪstrətəs/; Ancient Greek: Ἐρασίστρατος; c. 304 – c. 250 BC) was a Greek anatomist and royal physician under Seleucus I Nicator of Syria. Along with fellow physician Herophilus, he founded a school of anatomy in Alexandria, where they carried out anatomical research. As well, he is credited with helping to found the methodic school of teachings of medicine in Alexandria whilst opposing traditional humoral theories of Hippocratic ideologies. Together with Herophilus, he is credited by historians as the potential founder of neuroscience due to his acknowledgements of nerves and their roles in motor control through the brain and skeletal muscles.

Furthermore, Erasistratus is seen as one of the first physicians/scientists to conduct recorded dissections and potential vivisections alongside Herophilus. The two physicians were said by several Roman authors, notably Augustine, Celsus, and Tertullian, to have performed controversial vivisections on criminals to study the anatomy and possible physiology of human organs while they were in Alexandria. Because of their research, Erasistratus and Herophilus were heavily criticized for their utilization of vivisections; specifically, by the author Tertullian who followed Christian values. Erasistratus and Herophilus are thought to be the only physicians to perform systematic dissections on the human body until the Renaissance.

Erasistratus is credited for his description of the valves of the heart, and he also concluded that the heart was not the center of sensations, but that it instead functioned as a pump. He was among the first to distinguish between veins and arteries, believing that the arteries were full of air and that they carried the "animal spirit" (pneuma). He considered atoms to be the essential body element, and he believed they were vitalized by the pneuma that circulated through the nerves. He also thought that the nerves moved a nervous spirit from the brain. He then differentiated between the function of the sensory and motor nerves, and linked them to the brain. He is credited with one of the first in-depth descriptions of the cerebrum and cerebellum. Erasistratus is regarded by some as the founder of physiology.

Erasistratus is generally supposed to have been born at Ioulis on the island of Ceos, though Stephanus of Byzantium refers to him as a native of Cos; Galen, as a native of Chios; and the emperor Julian, as a native of Samos. Pliny says he was the grandson of Aristotle by his daughter Pythias, but this is not confirmed by any other ancient writer; and according to the Suda, he was the son of Cretoxena, the sister of the physician Medius, and Cleombrotus. From the latter it is not quite clear whether Cleombrotus was his father or his uncle. He was a pupil of Chrysippus of Cnidos, Metrodorus, and apparently Theophrastus.

He lived for some time at the court of Seleucus I Nicator, where he acquired great reputation by discovering the disease of Antiochus I Soter, the king's eldest son, probably 294 BC. Seleucus in his old age had lately married Stratonice, the young and beautiful daughter of Demetrius Poliorcetes, and she had already borne him one child. Antiochus fell violently in love with his stepmother, but did not disclose his passion, and chose rather to pine away in silence. The physicians were quite unable to discover the cause and nature of his disease, and Erasistratus himself was at a loss at first, till, finding nothing amiss about his body, he began to suspect that it must be his mind which was diseased, and that he might perhaps be in love. Erasistratus confirmed his conjecture when he observed that the skin of Antiochus grew hotter, his colour deeper, and his pulse quicker whenever Stratonice came near him, while none of these symptoms occurred on any other occasion. Accordingly, he told Seleucus that his son's disease was incurable, for he was in love, and that it was impossible to gratify his passion. The king wondered what the obstacle could be, and asked who the lady was. "My wife," replied Erasistratus; upon which Seleucus began to persuade him to give her up to his son. The physician asked him if he would do so himself if it were his wife that the prince was in love with. The king protested that he would most gladly; upon which Erasistratus told him that it was indeed his own wife who had inspired his passion, and that he chose rather to die than to disclose his secret. Seleucus was as good as his word, and not only gave up Stratonice, but also resigned to his son several provinces of his empire. This celebrated story is told with variations by many ancient authors, and a similar anecdote has been told of Hippocrates, Galen, Avicenna, and (if the names be not fictitious) Panacius and Acestinus. If this is the anecdote referred to by Pliny, as is probably the case, Erasistratus is said to have received one hundred talents for being the means of restoring the prince to health, which would amount to one of the largest medical fees upon record.

Very little more is known of the personal history of Erasistratus: he lived for some time at Alexandria, which was at that time beginning to be a celebrated medical school, and gave up practice in his old age, that he might pursue his anatomical studies without interruption. He and fellow physician Herophilus practiced anatomy with great success, and with such ardour that they are supposed to have dissected criminals alive. These criminals were supposedly supplied by the king at the request of Herophilus. By conducting these dissections on live subjects they were able to see the true color and shape of internal organs that were not present in deceased subjects. However, conducting these vivisections did not lead to the discovery that there was blood and not just pneuma present in the arteries, which should have been evident in dissecting a live person. Erasistratus appears to have died in Asia Minor, as the Suda mentions that he was buried by mount Mycale in Ionia. The exact date of his death is not known, but he probably lived to a good old age, as, according to Eusebius, he was alive 258 BC, about forty years after the marriage of Antiochus and Stratonice. He had numerous pupils and followers, and a medical school bearing his name continued to exist at Smyrna in Ionia nearly till the time of Strabo, about the beginning of the 1st century. The following are the names of the most celebrated physicians belonging to the sect founded by him: Apoemantes, Apollonius Memphites, Apollophanes Artemidoras, Athenion, Charidemus, Chrysippus, Heraclides of Smyrna, Hermogenes, Hicesius, Martialius, Menodorus, Ptolemaeus, Strato, Xenophon. An attack on Erasistratus and his followers is preserved in Anonymus Londinensis.

Erasistratus wrote many works on anatomy, practical medicine and pharmacy, of which only the titles remain, together with a great number of short fragments preserved by Galen, Caelius Aurelianus, and other ancient writers. These, however, are sufficient to enable us to form a reasonable idea of his opinions both as a physician and an anatomist. It is as an anatomist that he is most celebrated, and perhaps there is not one ancient physician that did more to promote that branch of medical science than he.

He appears to have been very near the discovery of the circulation of the blood, for in a passage preserved by Galen he says: