Polykleitos (/ˌpɔːliˈklaɪtoʊs/; Ancient Greek: Πολύκλειτος) was an ancient Greek sculptor, active in the 5th century BCE. Alongside the Athenian sculptors Pheidias, Myron and Praxiteles, he is considered as one of the most important sculptors of classical antiquity. The 4th century BCE catalogue attributed to Xenocrates (the "Xenocratic catalogue"), which was Pliny's guide in matters of art, ranked him between Pheidias and Myron. He is particularly known for his lost treatise, the Canon of Polykleitos (a canon of body proportions), which set out his mathematical basis of an idealised male body shape.

None of his original sculptures are known to survive, but many marble works, mostly Roman, are believed to be later copies.

His Greek name was traditionally Latinized Polycletus, but is also transliterated Polycleitus (Ancient Greek: Πολύκλειτος [polýkleːtos], "much-renowned") and, due to iotacism in the transition from Ancient to Modern Greek, Polyklitos or Polyclitus. He is called Sicyonius (lit. "The Sicyonian", usually translated as "of Sicyon") by Latin authors including Pliny the Elder and Cicero, and Ἀργεῖος (lit. "The Argive", trans. "of Argos") by others like Plato and Pausanias. He is sometimes called the Elder, in cases where it is necessary to distinguish him from his son, who is regarded as a major architect but a minor sculptor.

As noted above, Polykleitos is called "The Sicyonian" by some authors, all writing in Latin, and who modern scholars view as relying on an error of Pliny the Elder in conflating another more minor sculptor from Sikyon, a disciple of Phidias, with Polykleitos of Argos. Pausanias is adamant that they were not the same person, and that Polykleitos was from Argos, in which city state he must have received his early training, and a contemporary of Phidias (possibly also taught by Ageladas).

Polykleitos's figure of an Amazon for Ephesus was admired, while his colossal gold and ivory statue of Hera which stood in her temple—the Heraion of Argos—was favourably compared with the Olympian Zeus by Pheidias. He also sculpted a famous bronze male nude known as the Doryphoros ("Spear Bearer"), which survives in the form of numerous Roman marble copies. Further sculptures attributed to Polykleitos are the Discophoros ("Discus-bearer"), Diadumenos ("Youth tying a headband") and a Hermes at one time placed, according to Pliny, in Lysimachia (Thrace). Polykleitos's Astragalizontes ("Boys Playing at Knuckle-bones") was claimed by the Emperor Titus and set in a place of honour in his atrium. Pliny also mentions that Polykleitos was one of the five major sculptors who competed in the fifth century B.C. to make a wounded Amazon for the temple of Artemis; marble copies associated with the competition survive.

The statue of Diadumenos, also known as Youth Tying a Headband is one of Polykleitos's sculptures known from many copies. The gesture of the boy tying his headband represents a victory, possibly from an athletic contest. "It is a first-century A.D. Roman copy of a Greek bronze original dated around 430 B.C." Polykleitos sculpted the outline of his muscles significantly to show that he is an athlete. "The thorax and pelvis of the Diadoumenos tilt in opposite directions, setting up rhythmic contrasts in the torso that create an impression of organic vitality. The position of the feet poised between standing and walking give a sense of potential movement. This rigorously calculated pose, which is found in almost all works attributed to Polykleitos, became a standard formula used in Greco-Roman and, later, western European art."

Another statue created by Polykleitos is the Doryphoros, also called the Spear bearer. It is a typical Greek sculpture depicting the beauty of the male body. "Polykleitos sought to capture the ideal proportions of the human figure in his statues and developed a set of aesthetic principles governing these proportions that was known as the Canon or 'Rule'. He created the system based on mathematical ratios. "Though we do not know the exact details of Polykleitos’s formula, the end result, as manifested in the Doryphoros, was the perfect expression of what the Greeks called symmetria. On this sculpture, it shows somewhat of a contrapposto pose; the body is leaning most on the right leg. The Doryphoros has an idealized body, contains less of naturalism. In his left hand, there was once a spear, but if so it has since been lost. The posture of the body shows that he is a warrior and a hero. Indeed, some have gone so far as to suggest that the figure depicted was Achilles, on his way to the Trojan War, as a similar depiction of Achilles carrying a shield is seen on a vase painted by the Achilles Painter at around the same time.

Polykleitos, along with Phidias, created the Classical Greek style. Although none of his original works survive, literary sources identifying Roman marble copies of his work allow reconstructions to be made. Contrapposto, a pose that visualizes the shifting balance of the body as weight is placed on one leg, was a source of his fame.