Macedonian king of Egypt and coruler with his famous sister, Cleopatra VII (Cleopatra). He was killed while leading the Ptolemaic army against Julius Caesar (Caesar, Julius)'s forces in the final stages of the Alexandrian War.

A son of Ptolemy XII Auletes, Ptolemy XIII became joint ruler of Egypt with his sister following his father's death. In 49 Ptolemy, seeking to retain his father's allies, supplied the Roman general and former triumvir Pompey the Great with ships and troops. Subsequently, a court clique, headed by Theodotus, the eunuch Pothinus, and the general Achillas, gained influence over the king, fanning the growing rivalry between him and his strong-willed sister. Expelled from Egypt by the king and his clique in 48, she quickly raised an Arab army and besieged Pelusium, a city on the northeast frontier of Egypt. As the opposing forces prepared for war, Pompey, decisively defeated by Caesar at Pharsalus in Thessaly, appeared at Pelusium seeking refuge. He was murdered, however, on orders of the palace clique, which sought to gain favour with Caesar.

Shortly afterward, Caesar arrived at Alexandria and, seizing the palace quarter, ordered the warring factions to submit to his arbitration as authorized by the will of Ptolemy's father. Leaving General Achillas with the army, Ptolemy went with Pothinus to Caesar's camp, while Cleopatra arrived in the palace, reportedly concealed in a carpet. With all the members of the Ptolemaic royal family in his grasp, Caesar effected a reconciliation between Ptolemy and his sister.

Pothinus's group, however, continued to foment trouble against the Romans and their Egyptian allies; and after Achillas brought up the army to besiege Alexandria, Ptolemy's youngest sister, Arsinoe, escaped to the native forces. Caesar meanwhile persuaded Cleopatra to execute Pothinus, while Achillas was killed after feuding with Arsinoe, thus effectively destroying the clique. Pressed hard by the native forces under Arsinoe and her tutor, Caesar negotiated an exchange of Ptolemy for Arsinoe. The king immediately took command of the Egyptians; but Caesar, reinforced by an army from Pergamum, a city in Asia Minor, outmaneuvered the Ptolemaic forces, and the king was killed, probably by drowning as he attempted to flee.