Attalus I (Ancient Greek: Ἄτταλος 'Attalos'), surnamed Soter (Greek: Σωτήρ, 'Savior'; 269–197 BC), was the ruler of the Greek polis of Pergamon (modern-day Bergama, Turkey) and the larger Pergamene Kingdom from 241 BC to 197 BC. He was the adopted son of King Eumenes I, whom he succeeded, and the first of the Attalid dynasty to assume the title of king, sometime around 240 to 235 BC. He was the son of Attalus and his wife Antiochis.

Attalus won an important victory, the Battle of the Caecus River, over the Galatians, a group of migratory Celtic tribes from Thrace, who had been plundering and exacting tribute throughout most of Asia Minor for more than a generation. The victory was celebrated with a triumphal monument at Pergamon (The Dying Gaul) and Attalus taking the surname "Soter" and the title of king. He participated in the first and second Macedonian Wars against Philip V of Macedon as a loyal ally of the Roman Republic, although Pergamene participation was ultimately rather minor in these wars. He conducted numerous naval operations throughout the Aegean, gained the island of Aegina for Pergamon during the first war and Andros during the second, twice narrowly escaping capture at the hands of Philip V. During his reign, Pergamon also repeatedly struggled with the neighboring Seleucid Empire to the east, resulting in both successes and setbacks.

Attalus styled himself as a protector of the freedoms of the Greek cities of Anatolia as well as the champion of Greeks against barbarians. He funded art and monuments in Pergamon and in Greek cities he sought to cultivate as allies. He died in 197 BC at the age of 72, shortly before the end of the second war, having suffered an apparent stroke while addressing a Boeotian war council some months before. He and his wife Apollonis were admired for their rearing of their four sons. He was succeeded as king by his son Eumenes II.

Little is known about Attalus' early life. He was Greek and the son of Attalus and Antiochis. His father Attalus was the son of a brother (also called Attalus) of both Philetaerus, the founder of the Attalid dynasty, and Eumenes, the father of Eumenes I, Philetaerus' successor. The elder Attalus is recorded, along with his uncles, as providing generous donations to Delphi. His father also won fame as a charioteer, winning at Olympia, and was honored with a monument at Pergamon. It is conjectured the elder Attalus might have been considered a potential successor to Philetaerus, but Eumenes I succeeded to the throne instead. Attalus' mother Antiochis was probably related to the Seleucid royal family (perhaps a granddaughter of Seleucus I Nicator) with her marriage to Attalus' father likely arranged by Philetaerus to solidify his power.

At some point prior to 241 BC, Attalus' father died. If the elder Attalus had been heir designate at some point, he died before he could ever take the throne. The younger Attalus was adopted by Eumenes I, the incumbent dynast. After Eumenes' death in 241 BC, Attalus succeeded to the Pergamene throne.

Little is known of the early reign of Attalus. The main recorded event of the era was a battle with the Galatians. According to the 2nd century AD Greek writer Pausanias, "the greatest of his achievements" was the defeat of the "Gauls" (Γαλάται). The Galatians were immigrant Celts from Thrace, who had recently settled in Galatia in central Asia Minor, and whom the Romans and Greeks called Gauls, associating them with the Celts of what is now France, Switzerland, and northern Italy. Since the time of Philetaerus, the first Attalid ruler, the Galatians had posed a problem for Pergamon, indeed for all of Asia Minor, by exacting tributes to avoid war or other repercussions. Eumenes I had (probably), along with other rulers, dealt with the Galatians by paying these tributes. Attalus however refused to pay them, being the first such ruler recorded to do so. As a consequence, the Galatians set out to attack Pergamon, sometime around 238–235 BC. Attalus met them near the sources of the river Caicus and decisively won the resulting Battle of the Caecus River.

The prestige gained by the victory caused Attalus to take the surname of Soter, "savior", following the example of Antiochus I. He also declared himself basileus, king. While this did not increase his practical authority as his adopted father had already ruled like a king, it formally severed any relationship with the Seleucid Empire's king as a superior suzerain. The victory would be the core element of Attalus' reputation and fame. Attalus presented himself as the victorious champion of Greeks against barbarians, and commissioned much artwork and sculptures commemorating himself and the Pergamene victory.

As with other Attalid rulers, Pergamene royal coinage depicted a middle-aged version of Philetaerus, the dynasty's founder. Around the 230s BC, the depiction of Philetaerus changed from a plain band to a diadem entwined with a laurel wreath, the symbol of victory, perhaps to celebrate the defeat of the Galatians.