Ptolemy V Epiphanes Eucharistus (Greek: Πτολεμαῖος Ἐπιφανής Εὐχάριστος, Ptolemaĩos Epiphanḗs Eukháristos "Ptolemy the Manifest, the Beneficent"; 9 October 210–September 180 BC) was the King of Ptolemaic Egypt from July or August 204 BC until his death in 180 BC.

Ptolemy V, the son of Ptolemy IV and Arsinoe III, inherited the throne at the age of five when his parents died in suspicious circumstances. The new regent, Agathocles, was widely reviled and was toppled by a revolution in 202 BC, but the series of regents who followed proved incompetent and the kingdom was paralysed. The Seleucid king Antiochus III and the Antigonid king Philip V took advantage of the kingdom's weakness to begin the Fifth Syrian War (202–196 BC), in which the Ptolemies lost all their territories in Asia Minor and the Levant, as well as most of their influence in the Aegean Sea. Simultaneously, Ptolemy V faced a widespread Egyptian revolt (206–185 BC) led by the self-proclaimed pharaohs Horwennefer and Ankhwennefer, which resulted in the loss of most of Upper Egypt and parts of Lower Egypt as well.

Ptolemy V came of age in 196 BC and was crowned as pharaoh in Memphis, an occasion commemorated by the creation of the Rosetta Stone. After this, he made peace with Antiochus III and married the daughter of Antiochus III Cleopatra I in 194/3 BC. This outraged the Romans, who had entered into hostilities with Antiochus III partially on Ptolemy V's behalf, and after their victory they distributed the old Ptolemaic territories in Asia Minor to Pergamum and Rhodes rather than returning them to Egypt. However, Ptolemaic forces steadily reconquered the south of the country, bringing all of Upper Egypt back under Ptolemaic control in 186 BC. In his last years, Ptolemy V began manoeuvering for renewed warfare with the Seleucid empire, but these plans were cut short by his sudden death in 180 BC, allegedly poisoned by courtiers worried about the cost of the war.

Ptolemy V's reign saw greatly increased prominence of courtiers and the Egyptian priestly elite in Ptolemaic political life, a pattern that would continue for most of the rest of the kingdom's existence. It also marked the collapse of Ptolemaic power in the wider Mediterranean region. Arthur Eckstein has argued that this collapse sparked the "power transition crisis" that led to the Roman conquest of the eastern Mediterranean.

Ptolemy V was the only child of Ptolemy IV and his sister-wife Arsinoe III. The couple had come to power relatively young and ancient historiography remembered Ptolemy IV as being given over to luxury and ceremony, while leaving the government of Egypt largely to two courtiers, Sosibius and Agathocles (the latter being the brother of Ptolemy IV's concubine Agathoclea). In his early reign, Ptolemy IV successfully defeated the rival Seleucid empire in the Fourth Syrian War (219-217 BC), successfully preventing the Seleucid king Antiochus III from seizing Coele Syria for himself. His later reign, however, was troubled by native Egyptian revolts. Between 206 and 205 BC, Ptolemy IV lost control of Upper Egypt to the self-styled pharaoh Hugronaphor.

Ptolemy V was born in 210 BC, possibly on 9 October. He was made co-regent with his father shortly thereafter, probably on 30 November. In July or August of 204 BC, when Ptolemy V was five years old, his father and mother died in mysterious circumstances. It appears that there was a fire in the palace that killed Ptolemy IV, but it is unclear whether Arsinoe III also perished in this fire or was murdered afterwards to prevent her from becoming regent.

An uncertain amount of time elapsed after the death of Ptolemy IV and Arsinoe III (perhaps a week) during which Sosibius and Agathocles kept their deaths secret. Some time before September 204 BC, the royal bodyguard and army officers were gathered at the royal palace and Sosibius announced the death of the ruling couple and presented the young Ptolemy V to be acclaimed as king, wrapping the diadem around his head. Sosibius read out Ptolemy IV's will, which made Sosibius and Agathocles regents and placed Ptolemy V in the personal care of his mistress Agathoclea and her mother Oenanthe. Polybius thought that this will was a forgery produced by Sosibius and Agathocles themselves and modern scholars tend to agree with him. Sosibius is not heard of again after this event and it is generally assumed that he died. Hölbl suggests that the loss of his acumen was fatal to the regency.

Agathocles took a number of actions to solidify the new regime. Two months' pay were granted to the soldiers in Alexandria. Prominent aristocrats were dispatched overseas - to secure recognition of the succession from foreign powers and to prevent the aristocrats from challenging Agathocles for supremacy at home. Philammon, said to have carried out the murder of Arsinoe III, was sent to Cyrene as governor in order to assert Ptolemaic rule there. Pelops, governor of Cyprus, was sent to Antiochus III to ask him to continue to respect the peace treaty made with Ptolemy IV at the end of the Fourth Syrian War. Ptolemy, Sosibius' son, was sent to Philip V of Macedon to attempt to arrange an alliance against Antiochus III and a marriage between Ptolemy V and one of Philip V's daughters. Ptolemy of Megalopolis was sent to Rome, probably seeking support against Aniochus III. These missions were failures. Over the following year, Antiochus III seized Ptolemaic territory in Caria, including the city of Amyzon, and by late 203 BC he and Philip V had made a secret agreement to divide the Ptolemaic territories between themselves. War with Antiochus III was expected - Agathocles had also sent an embassy under Scopas the Aetolian to hire mercenaries in Greece in preparation for a conflict, although Polybius claims that his true purpose was to replace the Ptolemaic troops with mercenaries loyal to him.