Ptolemy III Euergetes (Greek: Πτολεμαῖος Εὐεργέτης, romanized: Ptolemaîos Euergétēs, "Ptolemy the Benefactor"; c. 280 – November/December 222 BC) was the third pharaoh of the Ptolemaic dynasty in Egypt from 246 to 222 BC. The Ptolemaic Kingdom reached the height of its military and economic power during his kingship, as initiated by his father Ptolemy II Philadelphus.

Ptolemy III was the eldest son of Ptolemy II and Arsinoe I. When Ptolemy III was young, his mother was disgraced and he was removed from the succession. He was restored as heir to the throne in the late 250s BC and succeeded his father as king without issue in 246 BC. On his succession, Ptolemy III married Berenice II, reigning queen of Cyrenaica, thereby bringing her territory into the Ptolemaic realm. In the Third Syrian War (246–241 BC), Ptolemy III invaded the Seleucid empire and won a near total victory, but was forced to abandon the campaign as a result of an uprising in Egypt. In the aftermath of this rebellion, Ptolemy forged a closer bond with the Egyptian priestly elite, which was codified in the Canopus decree of 238 BC and set a trend for Ptolemaic power in Egypt for the rest of the dynasty. In the Aegean, Ptolemy III suffered a major setback when his fleet was defeated by the Antigonids at the Battle of Andros around 246 BC, but he continued to offer financial support to their opponents in mainland Greece for the rest of his reign. At his death, Ptolemy III was succeeded by his eldest son, Ptolemy IV.

Ptolemy III was born some time around 280 BC, as the eldest son of Ptolemy II and his first wife Arsinoe I, daughter of King Lysimachus of Thrace. His father had become co-regent of Egypt in 284 BC and sole ruler in 282 BC. Around 279 BC, the collapse of Lysimachus' kingdom led to the return to Egypt of Ptolemy II's sister Arsinoe II, who had been married to Lysimachus. A conflict quickly broke out between Arsinoe I and Arsinoe II. Sometime after 275 BC, Arsinoe I was charged with conspiracy and exiled to Coptos. When Ptolemy II married Arsinoe II probably in 273/2 BC, her victory in this conflict was complete. As children of Arsinoe I, Ptolemy III and his two siblings seem to have been removed from the succession after their mother's fall. This political background may explain why Ptolemy III seems to have been raised on Thera in the Aegean, rather than in Egypt. His tutors included the poet and polymath Apollonius of Rhodes, later head of the Library of Alexandria.

From 267 BC, a figure known as Ptolemy "the Son" was co-regent with Ptolemy II. He led naval forces in the Chremonidean War (267–261 BC), but revolted in 259 BC at the beginning of the Second Syrian War and was removed from the co-regency. Some scholars have identified Ptolemy the Son with Ptolemy III. This seems unlikely, since Ptolemy III was probably too young to lead forces in the 260s and does not seem to have suffered any of the negative consequences that would be expected if he had revolted from his father in 259 BC. Chris Bennett has argued that Ptolemy the Son was a son of Arsinoe II by Lysimachus. Around the time of the rebellion, Ptolemy II legitimised the children of Arsinoe I by having them posthumously adopted by Arsinoe II.

In the late 250s BC, Ptolemy II arranged the engagement of Ptolemy III to Berenice, the sole child of Ptolemy II's half-brother King Magas of Cyrene. The decision to single Ptolemy III out for this marriage indicates that, by this time, he was the heir presumptive. On his father's death, Ptolemy III succeeded him without issue, taking the throne on 28 January 246 BC.

Cyrene had been the first Ptolemaic territory outside Egypt, but Magas had rebelled against Ptolemy II and declared himself king of Cyrenaica in 276 BC. The aforementioned engagement of Ptolemy III to Berenice had been intended to lead to the reunification of Egypt and Cyrene after Magas' death. However, when Magas died in 250 BC, Berenice's mother Apame refused to honour the agreement and invited an Antigonid prince, Demetrius the Fair, to Cyrene to marry Berenice instead. With Apame's help, Demetrius seized control of the city, but he was assassinated by Berenice. A republican government, led by two Cyrenaeans named Ecdelus and Demophanes, controlled Cyrene for four years.

It was only with Ptolemy III's accession in 246 BC, that the wedding of Ptolemy III and Berenice seems to have actually taken place. Ptolemaic authority over Cyrene was forcefully reasserted. Two new port cities were established, named Ptolemais and Berenice (modern Tolmeita and Benghazi) after the dynastic couple. The cities of Cyrenaica were unified in a League overseen by the king, as a way of balancing the cities' desire for political autonomy against the Ptolemaic desire for control.

In July 246 BC, Antiochus II, king of the Seleucid empire, died suddenly. By his first wife Laodice I, Antiochus II had had a son, Seleucus II, who was about 19 years old in 246 BC. However, in 253 BC, he had agreed to repudiate Laodice and marry Ptolemy III's sister Berenice. Antiochus II and Berenice had a son named Antiochus, who was still an infant when his father died. A succession dispute broke out immediately after Antiochus II's death. Ptolemy III quickly invaded Syria in support of his sister and her son, marking the beginning of the Third Syrian War (also known as the Laodicean War).