Berenice II Euergetis (267 or 266 – 221 BCE; Greek: Βερενίκη Ευεργέτις, Berenikē Euergetis, "Berenice the Benefactress") was queen regnant of Cyrenaica from 258 to 246 BCE and queen of Ptolemaic Egypt from 246 to 222 BCE as the wife of Ptolemy III. She is sometimes considered co-regent of her Ptolemaic husband.

She married Demetrius the Fair, thus giving him the throne of Cyrenaica, on the death of her father Magas in 250/249 BCE. After a short power struggle with her mother, Berenice married her half-cousin Ptolemy III, the third ruler of the Ptolemaic kingdom. This marriage led to the re-incorporation of Cyrenaica into the Ptolemaic empire. As queen of Egypt, Berenice participated actively in government, was incorporated into the Ptolemaic state cult alongside her husband and worshipped as a goddess in her own right. She is best known for sacrificing her hair as a votive offering, which led to the constellation Coma Berenices being named after her. Berenice was murdered by the regent Sosibius shortly after the accession of her son Ptolemy IV Philopator in 221 BCE.

Cyrenaica had been incorporated into the Ptolemaic realm in 323 BCE, by Ptolemy I Soter shortly after the death of Alexander the Great. The region proved difficult to control and around 300 BCE, Ptolemy I entrusted the region to Magas, son of his wife Berenice I by an earlier marriage. After Ptolemy I's death, Magas asserted his independence and engaged in warfare with his successor Ptolemy II Philadelphus. Around 275 BCE, Magas married Apama, who came from the Seleucid dynasty, which had become enemies of the Ptolemies. Berenice II was their only child. When Ptolemy II renewed his efforts to reach a settlement with Magas of Cyrene in the late 250s BCE, it was agreed that Berenice would be married to her half-cousin, the future Ptolemy III, who was Ptolemy II's heir.

The astronomer Gaius Julius Hyginus claims that when Berenice's father and his troops were routed in battle, Berenice mounted a horse, rallied the remaining forces, killed many of the enemy, and drove the rest to retreat. The veracity of this story is unclear and the battle in question is not otherwise attested, but "it is not on the face of it impossible."

Berenice was hailed basilissa (queen) on coins even in her father's lifetime. There are Cyrenean coins with the portrait of queen, the legend ΒΕΡΕΝΙΚΗΣ ΒΑΣΙΛΙΣΣΗΣ (Berenice Basilissa), and the monogram of Magas. It is evidently more plausible that the queen's identity is Magas's daughter Berenice II rather than Magas's mother Berenice I, because the portrait is youthful and unveiled, meaning unmarried. According to coins of Berenice, the accession of Berenice as queen of Cyrene was in 258 BCE.

King Magas died in circa 250 BCE. At this point, Berenice's mother Apama refused to honour the marriage agreement with the Ptolemies and invited an Antigonid prince, Demetrius the Fair to Cyrene to marry Berenice instead. With Apama's help, Demetrius seized control of the city. Allegedly, Demetrius and Apama became lovers. Berenice is said to have discovered them in bed together and had him assassinated. Apama was spared. Control of Cyrene was then entrusted to a republican government, led by two Cyrenaeans named Ecdelus and Demophanes, until Berenice's actual wedding to Ptolemy III in 246 BCE after his accession to the throne. It seems most probable that Berenice conceded a certain degree of autonomy to Cyrene.

Berenice married Ptolemy III in 246 BCE after his accession to the throne. This brought Cyrenaica back into the Ptolemaic realm, where it would remain until her great-great-grandson Ptolemy Apion left it to the Roman Republic in his will in 96 BCE.

In 244 or 243 BCE, Berenice and her husband were incorporated into the Ptolemaic state cults and worshipped as the Theoi Euergetai (Benefactor Gods), alongside Alexander the Great and the earlier Ptolemies. Berenice was also worshipped as a goddess on her own, Thea Euergetis (Benefactor Goddess). She was often equated with Aphrodite and Isis and came to be particularly associated with protection against shipwrecks. Most of the evidence for this cult derives from the reign of Ptolemy IV or later, but a cult in her honour is attested in the Fayyum in Ptolemy III's reign. This cult closely parallels that offered to her mother-in-law, Arsinoe II, who was also equated with Aphrodite and Isis, and associated with protection from shipwrecks. The parallelism is also presented on the gold coinage minted posthumously in honour of the two queens. The coinage of Arsinoe II bears a pair of cornucopiae on the reverse side, while that of Berenice bears a single cornucopia.