Polycrates (/pəˈlɪkrəˌtiːz/; Ancient Greek: Πολυκράτης), son of Aeaces, was the tyrant of Samos from the 540s BC to 522 BC. He had a reputation as both a fierce warrior and an enlightened tyrant.

The main source for Polycrates' life and activities is the historian Herodotus, who devotes a large section of book 3 of his Histories to the rise and fall of Polycrates (3.39-60, 3.120-126). His account was written in the third quarter of the 5th century BC, nearly a century after Polycrates' death, was based mostly on oral traditions and incorporates many folk-tale elements. Furthermore, Herodotus creatively shaped his account of Polycrates in order to make general moral points and to comment on the imperialism of the Athenian empire in his own day. Some poetry from Polycrates' time comments on him in passing and there is a smattering of references to Polycrates in other literary sources ranging in date from the 4th century BC to the Roman Imperial period. These sources preserve useful information but tend to assimilate Polycrates to a stereotypical model of the tyrannical ruler, which may be anachronistic.

Polycrates' family background is not clearly known to us. J.P. Barron proposed that Polycrates' ancestors formed a dynasty that ruled Samos from around 600 BC. A shadowy figure, Syloson, son of Calliteles might have been the founder of this dynasty. Barron further proposed that Polycrates' father, Aeaces, ruled Samos around the middle of the sixth century. An inscription survives from this period, in which an individual called Aeaces dedicates some plunder to Hera. Barron's analysis was broadly accepted by Graham Shipley, but has been challenged by Aideen Carty.

Polycrates had two brothers, Pantagnotus and Syloson, who were originally his co-rulers. Syloson ruled Samos again after Polycrates' death, and was succeeded by his own son, Aeaces. Herodotus mentions a daughter in his account of Polycrates' death.^{[full citation needed]}

In the mid-sixth century BCE, there was apparently a period of civil strife in Samos. This conflict is mentioned by Herodotus in the context of Polycrates' rise to power. John Boardman and Graham Shipley have cited archaeological evidence for serious disruption in this period. Around 550 BC, they say, funerary stele were shattered and aristocratic burials in the West Cemetery at Samos cease, while the first great temple of Hera, known as the Rhoikos temple, was destroyed - only a decade after it was built. However, more recent archaeological research has challenged this picture, showing that the destruction of the Rhoikos temple was a structural failure resulting from subsidence under the foundations, and that the West Cemetery fell out of use gradually over the course of the second half of the sixth century. Aideen Carty argues that shifts in the deposition of Laconian pottery on Samos suggest the development of pro- and anti-Spartan factions on the island, one faction associated with the Heraion and the other with the Artemision of Samos.

Herodotus reports that Polycrates took power with his brothers Pantagnotus and Syloson and a force of only fifteen men. This coup seems to have taken place in 540 BC or slightly earlier. Initially, Polycrates ruled along with his brothers, but soon had Pantagnotus killed and then exiled Syloson to take full control for himself. According to a Roman-period author, Polyaenus, there was a religious procession in armour out of the city of Samos to the Heraion, led by Pantagnotus and Syloson. When the procession was over, the Samians removed their armour to sacrifice, and Pantagnotus and Syloson seized the chance to murder their enemies. At the same time, Polycrates seized Astypalaea, the citadel of the city of Samos. The tyrant of Naxos, Lygdamis, invaded with a force to support Polycrates. However, it is uncertain whether Polyaenus' account describes Polycrates' initial seizure of power or the conflict with his brothers which left him as sole ruler.

Polycrates recruited an army of 1,000 archers and assembled a navy of 100 penteconters, which became the most powerful navy in the Greek world –– Herodotus says that Polycrates was the first Greek ruler to understand the importance of sea power and Thucydides includes him in his list of thalassocracies in the Aegean. With these forces he implemented a plan to bring all the Greek islands and cities of Ionia under his rule. Polycrates' rise to power took place in the period when the Achaemenid empire under Cyrus conquered western Anatolia. In theory, the Aegean islanders had accepted Persian overlordship after Cyrus conquered Lydia in 546 BC, but in practice the political situation in the Aegean was complicated. This confusion may have contributed to Polycrates' success in projecting his power.

Few specifics of Polycrates' naval activities and conquests are preserved. Herodotus refers to an attack on Miletus, in which the Lesbians came to the aid of Miletos and Polycrates won a great naval victory, capturing and enslaving large numbers of Lesbian sailors. The Milesians had become key Persian allies and this victory is probably identical with a victory over Persian navy mentioned in Thucydides and some other historians. Thucydides emphasises his conquest of Delos, the key religious centre of the Aegean. Polycrates formed an alliance with King Amasis of Egypt and A. Carty suggests that Polycrates assisted Amasis in the conquest of Cyprus. In general, though, the ancient sources stress not specific campaigns, but wide-ranging raiding, which may have been more like piracy than campaigns of conquest. Herodotus says that he "raided everyone without any discrimination. For he said that a friend would be more appreciative if what was taken from him was returned than if it had not first been snatched away." Aideen Carty argues that the focus of this raiding was the acquisition of slaves whom he exported to Egypt to serve as mercenaries in Amasis' army.