Philetaerus (Greek: Φιλέταιρος, Philétairos, ca. 343 BC–263 BC) was the founder of the Attalid dynasty of Pergamon in Anatolia.
He was born in Tieium (Greek: Tieion), a small town on the Black Sea coast of Anatolia between Bithynia to the west and Paphlagonia to the east. His father was Attalus (Greek: Attalos) (perhaps from Macedon) and his mother Boa was Paphlagonian.
After the death of Alexander the Great in 323 BC, Philetaerus became embroiled in the struggle for supremacy, called the Wars of the Diadochi (diadochi means "successors" in Greek) between Alexander's regional governors, Antigonus in Phrygia, Lysimachus in Thrace and Seleucus in Babylonia (among others). Philetaerus served first under Antigonus. He then shifted his allegiance to Lysimachus (ruler of Pergamon from 323 BC to 281 BC), who, after Antigonus was killed at the Battle of Ipsus in 301 BC, made Philetaerus commander of Pergamon, where Lysimachus kept a treasury of nine thousand talents of silver.
Philetaerus served Lysimachus until 282 BC, when perhaps because of conflicts involving the court intrigues of Arsinoe, Lysimachus' third wife, Philetaerus deserted Lysimachus, offering himself and the important fortress of Pergamon, along with its treasury to Seleucus, who subsequently defeated and killed Lysimachus at the Battle of Corupedium in 281 BC. Seleucus himself was killed by Ptolemy Ceraunus, a brother of Arsinoe at Lysimachia a few months later.
Though nominally under Seleucid control, Philetaerus, especially after the death of Seleucus, had considerable autonomy and was able with the help of his considerable wealth to increase his power and influence beyond Pergamon. There are numerous records of Philetaerus as benefactor to neighboring cities and temples, including the temples at Delphi and Delos. He also contributed troops, money and food to the city of Cyzicus for defense against the invading Gauls. As a result Philetaerus gained prestige and goodwill for himself and his family.
During his nearly forty year rule, he constructed on the acropolis of Pergamon, the temple of Demeter, and the temple of Athena (Pergamon's patron deity), and Pergamon's first palace and he added considerably to the city's fortifications.
Philetaerus was a eunuch, though scholars differ on the reason for his castration. Attalus I, the first Attalid king of Pergamon, explained that when Philetaerus was a baby, he was brought into a crowd where he was pressed upon and his testicles were crushed. Some scholars believe that this story was concocted by Attalus as a way to make the origins of his dynasty look better, for eunuchs that were castrated for the purpose of royal service (which is the other plausible reason for Philetaerus' eunuchism) were often humiliated. The fact that Philetaerus was a eunuch can be seen by his overweight appearance on coins minted after his death.
"Philetaerus of Tieium, was a eunuch from boyhood; for it came to pass at a certain burial, when a spectacle was being given at which many people were present, that the nurse who was carrying Philetaerus, still an infant, was caught in the crowd and pressed so hard that the child was incapacitated. He was a eunuch, therefore, but he was well trained and proved worthy of this trust."
Philetaerus never married and, since he was a eunuch, had no children. He adopted his nephew Eumenes I (the son of Philetaerus' brother also named Eumenes), who succeeded him as ruler of Pergamon, upon his death in 263 BC. With the exception of Eumenes II, all future Attalid rulers depicted the bust of Philetaerus on their coins, paying tribute to the founder of their dynasty.
（Φιλέταιρος). The founder of the kingdom of Pergamum, a native of Paphlagonia (Strabo, pp. 543, 623). He had served in the army of Antigonus and later in that of Lysimachus, who put him in charge of the treasure stored at Pergamum. Philetaerus, shifting for himself, declared in favour of Seleucus (q.v.), but after the death of the latter (B.C. 280) practically established his own independence, and on his death (B.C. 263) left the government to his nephew Eumenes (Lucian, Macrob. 12).