Aelius Galenus or Claudius Galenus (Greek: Κλαύδιος Γαληνός; September 129 – c. 216 AD), often anglicized as Galen (/ˈɡeɪlən/) or Galen of Pergamon, was a Roman and Greek physician, surgeon, and philosopher. Considered to be one of the most accomplished of all medical researchers of antiquity, Galen influenced the development of various scientific disciplines, including anatomy, physiology, pathology, pharmacology, and neurology, as well as philosophy and logic.

The son of Aelius Nicon, a wealthy Greek architect with scholarly interests, Galen received a comprehensive education that prepared him for a successful career as a physician and philosopher. Born in the ancient city of Pergamon (present-day Bergama, Turkey), Galen traveled extensively, exposing himself to a wide variety of medical theories and discoveries before settling in Rome, where he served prominent members of Roman society and eventually was given the position of personal physician to several emperors.

Galen's understanding of anatomy and medicine was principally influenced by the contemporary theory of the four humors: black bile, yellow bile, blood, and phlegm, as first advanced by the author of On the Nature of Man in the Hippocratic corpus. Galen's views dominated and influenced Western medical science for more than 1,300 years. His anatomical reports were based mainly on the dissection of Barbary apes. However, while dissections and vivisections on humans were practiced in Alexandria by Herophilus and Erasistratus in the 3rd century BCE under Ptolemaic permission, by Galen's time these procedures were strictly forbidden in the Roman Empire. As Galen discovered that the facial expressions of the Barbary apes were particularly vivid, Galen switched to pigs for his research to avoid prosecution. Aristotle had used pigs centuries earlier for his study of anatomy and physiology. Galen, like others, reasoned that animal anatomy had a strong conciliance with that of humans. Galen would encourage his students to go look at dead gladiators or bodies that washed up in order to get better acquainted with the human body.

Galen's theory of the physiology of the circulatory system remained unchallenged until c. 1242, when Ibn al-Nafis published his book Sharh tashrih al-qanun li' Ibn Sina (Commentary on Anatomy in Avicenna's Canon), in which he reported his discovery of pulmonary circulation. His anatomical reports remained uncontested until 1543, when printed descriptions and illustrations of human dissections were published in the seminal work De humani corporis fabrica by Andreas Vesalius, where Galen's physiological theory was accommodated to these new observations.

Galen saw himself as both a physician and a philosopher, as he wrote in his treatise titled That the Best Physician Is Also a Philosopher. Galen was very interested in the debate between the rationalist and empiricist medical sects, and his use of direct observation, dissection, and vivisection represents a complex middle ground between the extremes of those two viewpoints. Many of his works have been preserved or translated from the original Greek, although many were destroyed and some credited to him are believed to be spurious. Although there is some debate over the date of his death, he was no younger than seventy when he died.

Galen's Greek name Γαληνός (Galēnós) comes from the adjective γαληνός (galēnós) 'calm'. Galen's Latin name (Aelius or Claudius) implies he had Roman citizenship.

Galen describes his early life in On the affections of the mind. He was born in September 129 AD. His father, Aelius Nicon, was a wealthy patrician, an architect and builder, with eclectic interests including philosophy, mathematics, logic, astronomy, agriculture and literature. Galen describes his father as a "highly amiable, just, good and benevolent man". At that time Pergamon (modern-day Bergama, Turkey) was a major cultural and intellectual centre, noted for its library, second only to that in Alexandria, as well as being the site of a large temple to the healing god Asclepius. The city attracted both Stoic and Platonic philosophers, to whom Galen was exposed at age 14. His studies also took in each of the principal philosophical systems of the time, including Aristotelian and Epicurean. His father had planned a traditional career for Galen in philosophy or politics and took care to expose him to literary and philosophical influences. However, Galen states that in around 145 his father had a dream in which the god Asclepius appeared and commanded Nicon to send his son to study medicine.

Following his earlier liberal education, Galen at age 16 began his studies at the prestigious local healing temple or asclepeion as a θεραπευτής (therapeutes, or attendant) for four years. There he came under the influence of men like Aeschrion of Pergamon, Stratonicus and Satyrus. Asclepiea functioned as spas or sanitoria to which the sick would come to seek the ministrations of the priesthood. Romans frequented the temple at Pergamon in search of medical relief from illness and disease. It was also the haunt of notable people such as the historian Claudius Charax, the orator Aelius Aristides, the sophist Polemo, and the consul Cuspius Rufinus.