Philip III Arrhidaeus (; BC – 317 BC) was king of the ancient Greek kingdom of Macedonia from 323 until his execution in 317 BC. He was a son of King Philip II of Macedon by Philinna of Larissa, and thus an elder half-brother of Alexander the Great. Named Arrhidaeus at birth, he assumed the name Philip when he ascended to the throne.

As Arrhidaeus grew older it became apparent that he had mild learning difficulties. Plutarch was of the view that he became disabled by means of an attempt on his life by Philip II's wife, Queen Olympias, who wanted to eliminate a possible rival to her son, Alexander, through the employment of pharmaka (drugs/spells); however, most modern authorities doubt the truth of this claim.

Alexander was fond of Arrhidaeus and took him on his campaigns, both to protect his life and to prevent his use as a pawn in any prospective challenge for the throne. After Alexander's death in Babylon in 323 BC, the Macedonian army in Asia proclaimed Arrhidaeus as king; however, he served merely as a figurehead and as the pawn of a series of powerful generals.

Arrhidaeus's whereabouts during the reign of his brother Alexander are unclear from the extant sources; what is certain is that no civil or military command was given to him in those thirteen years (336–323 BC).

Succession

Arrhidaeus was in Babylon at the time of Alexander's death on 10 June 323 BC. A succession crisis ensued. Arrhidaeus was the most obvious candidate, but he was mentally disabled and thus unfit to rule. A conflict then arose between Perdiccas, leader of the cavalry, and Meleager, who commanded the phalanx: the first wanted to wait to see if Roxana, Alexander's pregnant wife, would deliver a son while Meleager objected that Arrhidaeus should be chosen king. A compromise was engineered in which Arrhidaeus would become king with the name of Philip III, and would be joined by Roxana's yet-unborn child as co-sovereign should that child prove to be a male. This eventuality did indeed arise and resulted in Roxana's son, Alexander, becoming with his uncle Phillip co-sovereign on the throne of Macedon. It was immediately decided that Philip Arrhidaeus would reign, but not rule: this was to be the prerogative of the new regent, Perdiccas.

When news arrived in Macedonia that Arrhidaeus had been chosen as king, Cynane, a daughter of Philip II, developed a plan to travel to Asia and offer the new king her daughter Eurydice for his wife. This move was an obvious affront to the regent, whom Cynane had completely bypassed, and to prevent the marriage, Perdiccas sent his brother, Alcetas, to kill Cynane. The reaction among the troops generated by this murder was such that the regent had to give up his opposition to the proposed match and accept the marriage. From that moment on, Philip Arrhidaeus was to be under the sway of his bride, a proud and determined woman bent on substantiating her husband's power.

Regents

thumb|350px|Coin of Philip III Arrhidaios. 323–317 BC. AR Tetradrachm (17.20 g, 1h). Babylon mint. Struck under [[Perdikkas, circa 323–320 BC. Head of Herakles left, wearing lion skin headdress / [BASILEWS FILIPPOU], Zeus Aëtophoros seated right; wheel and monogram in left field, monogram below throne.]]

Eurydice's chance to increase her husband's power came when the first war of the Diadochi sealed the fate of Perdiccas, making a new settlement necessary. An agreement was made at Triparadisus in Syria in 321 BC. Eurydice moved deftly enough to achieve the removal of the first two designated regents, Peithon and Arrhidaeus (a namesake of her husband), but was powerless to block the aspirations of Antipater, whose position proved too powerful, and the latter was made the new regent; Philip Arrhidaeus and Eurydice were forced to follow Antipater back to Macedonia.

The regent died of natural causes the following year, nominating as his successor not his son Cassander, but his friend and lieutenant, Polyperchon. Cassander's refusal to accept his father's decision sparked the Second War of the Diadochi, in which Eurydice saw once again a chance to free Philip from the control of the regent.

An opportunity presented itself in 317 BC when Cassander expelled Polyperchon from Macedonia. Eurydice immediately allied herself with Cassander and persuaded her husband to nominate him as the new regent. Cassander reciprocated by leaving her in full control of the country when he left to campaign in Greece.

Death

That same year (317 BC), Polyperchon and Olympias allied with her cousin, Aeacides, king of Epirus, and invaded Macedonia. The Macedonian troops refused to fight Olympias, the mother of Alexander. Philip and Eurydice had no choice but to escape, only to be captured at Amphipolis and thrown into prison.

It soon became clear that Philip was too dangerous to be left alive, as Olympias's many enemies saw him as a useful tool against her. She had him executed, while his wife was forced to commit suicide.

The lunar crater Ariadaeus was named after him.

Tomb

240px|thumb|right|[[Larnax|Golden Larnax (Chrysi Larnaka) (with the Sun of Vergina on the lid) that contains the remains (bones) from the burial of King Philip II of Macedonia and the royal golden wreath. Formerly located at the Thessaloniki Archaeological Museum, since 1997 displayed in the underground museum of Vergina, inside the Great Tumulus.]]

In 1977, excavations near Vergina in northern Greece led to the unearthing of three Macedonian tombs. Tomb II is a two-chambered royal tomb richly decorated with grave goods such as silver vessels, bronze weaponry and bath equipment, and a gold-and-ivory shield. The outer chamber of the tomb contained a golden box with the bones of a woman in her twenties, and the inner chamber contained a gold box with a golden crown and the bones of a man in his forties.

Due to forensic studies in 2015, it is now believed that Philip II was buried in Tomb I. As such, others have proposed Tomb II to contain the remains of Philip Arrhidaeus and his wife Eurydice, due to the time of their death in 317 BC and that Arrhidaeus was known to have received honors at burial. It is also possible that some of Alexander the Great's armor could have been buried at the tomb, as Arrhidaeus wore Alexander's garments when he ascended to the throne in 323 BC, although he did not fight in battles himself. A hand-hammered iron helmet found in Tomb II matches Plutarch's description of Alexander's helmet.