The Kingdom of Pergamon, Pergamene Kingdom, or Attalid kingdom was a Greek state during the Hellenistic period that ruled much of the Western part of Asia Minor (now Turkey) from its capital city of Pergamon. It was ruled by the Attalid dynasty (/ˈætəlɪd/; Greek: Δυναστεία των Ατταλιδών, romanized: Dynasteía ton Attalidón).

The kingdom was a rump state that was created from the territory ruled by Lysimachus, a general of Alexander the Great. Philetaerus, one of Lysimachus' lieutenants, rebelled and took the city of Pergamon and its environs with him; Lysimachus died soon after in 281 BC. The new kingdom was initially in a vassal-like relationship of nominal fealty to the Seleucid Empire, but exercised considerable autonomy and soon became entirely independent. It was a monarchy ruled by Philetaerus's extended family and their descendants. It lasted around 150 years before being eventually absorbed by the Roman Republic during the period from 133–129 BC.

Philetaerus rose from humble origins to become a lieutenant of Lysimachus, one of Alexander the Great's generals (diadochi), who ruled a large state centered around Byzantium. Philetaerus was trusted to manage the fortress of Pergamon and guard much of Lysimachus's treasury, and had 9,000 talents under his purview. At some point prior to 281 BC, Philetaerus deserted Lysimachus and rebelled, allegedly over fears of Arsinoe, Lysimachus's wife, who was accused of arranging the death of Agathocles, Lysimachus's son. In 281 BC, Seleucus I Nicator, another of Alexander's generals, defeated and killed Lysimachus at the Battle of Corupedium, while Seleucus himself was killed a few months later. Philetaerus offered his services to Seleucus and his successors of the Seleucid Empire, but enjoyed considerable autonomy. He extended his power and influence beyond just the city of Pergamon, making allies with neighboring city states. He contributed troops, money, and food to the city of Cyzicus, in Mysia, for its defense against the invading Gauls, thus gaining prestige and goodwill for him and his family. He built the sanctuary of Demeter on the acropolis of Pergamon, the temple of Athena (Pergamon's patron deity), and Pergamon's first palace. He added considerably to the city's fortifications.

Philetaerus' nephew and adopted son, Eumenes I, succeeded him upon his death in 263 BC. He rebelled and defeated the Seleucid king Antiochus I Soter near the Lydian capital of Sardis in 261 BC. He created an outright independent Pergamene state, and greatly increased its territories. He established garrisons, such as Philetaireia, in the north at the foot of Mount Ida, which was named after his adoptive father, and Attaleia, in the east, to the northeast of Thyatira near the sources of the river Lycus, which was named after his grandfather. He also extended his control to the south of the river Caïcus, reaching the Gulf of Cyme. Eumenes I minted coins with the portrait of Philetaerus, who during his reign had still been depicting the Seleucid king Seleucus I Nicator on his coins.

Attalus I (r. 241–197 BC) succeeded Eumenes I after being adopted as his son. Early in his reign, he won a battlefield victory against the Galatians of Asia Minor (called Gauls by Pausanias) at the Battle of the Caecus River. This victory was a key to the legitimacy of Hellenistic kings, who styled themselves after Alexander the Great's legacy of military glories, and improved the standing and prestige of the kingdom. Attalus took the name Soter, "Savior", afterward, and explicitly took the title of basileus, king. Several years later, the "War of the Brothers" broke out in the Seleucid Empire between Seleucus II Callinicus and Antiochus Hierax. Antiochus Hierax made alliances with other kings in Asia Minor, his base of power, including both the Galatians and the Cappadocians. Around 230 BC, Hireax attacked Pergamon with the help of the Galatians. Attalus defeated the Gauls and Antiochus in the Battle of Aphrodisium and in a second battle in the east. He then fought Antiochus alone in a battle near Sardis and in the Battle of the Harpasus in Caria in 229 BC. After this Antiochus left to start a campaign in Mesopotamia, and then pivoted toward Thrace in 227 BC. He was killed in battle against the Gauls and the Kingdom of Tylis. With Antiochus Hierax's death, Attalus gained control over all Seleucid territories in Asia Minor north of the Taurus Mountains. He repulsed several attempts by Seleucus III Ceraunus, who had succeeded Seleucus II, to recover the lost territory. The newly expanded kingdom stretched over 143,000 square kilometres (55,000 sq mi).

The expansion was not to last long. In 223 BC, Seleucus III crossed the Taurus, but was assassinated, and the general Achaeus assumed control of the Seleucid army. Antiochus III the Great made Achaeus governor of the Seleucid territories north of the Taurus. Achaeus embarked upon a remarkably successful military campaign. Within two years, he had recovered the lost territories, taken parts of the traditional Pergamene heartland, and forced Attalus to retreat within the walls of Pergamon. However, Achaeus himself turned on Antiochus III and proclaimed himself a king, perhaps because he was accused of intending to revolt anyway, or perhaps simply drunk with success. By 220/219 BC, Achaeus and Attalus seem to have made peace.

In 218 BC, Achaeus undertook an expedition to Selge, south of the Taurus. Attalus recaptured his former territories with the help of some Thracian Gauls. Achaeus returned from his victorious campaign in 217 BC and hostilities between the two resumed. Attalus made an alliance with Antiochus III, who besieged Achaeus in Sardis in 214 BC. Antiochus captured the city and put Achaeus to death in the next year. Attalus regained control over his territories.

The Attalids became allies of the Roman Republic during the First Macedonian War (214–205 BC), although their participation was rather ineffective and insignificant. They would go on to support Rome in many subsequent wars. Attalus I, who had helped the Romans in the first war, also provided them with assistance in the Second Macedonian War (200–197 BC).