Achillas (died 47 BCE) was a general during the reign of Ptolemy XIII.
Achillas was a member of Ptolemy's inner circle of advisors along with Pothinus, Theodotus of Chios and Lucius Septimius. Pothinus and Achillas would often interfere in the rule of Ptolemy and his sister, Cleopatra, eventually turning the king against his co-ruler and forcing her into exile.
In 48 BCE, the Roman general Julius Caesar arrived in Alexandria to arbitrate the dispute between Ptolemy and Cleopatra over the rule of Egypt, and he ordered the siblings to rule jointly. However, Pothinus, a member of the Order of the Ancients, had no intention of following Caesar's commands, wishing to use Ptolemy as a puppet ruler without Cleopatra's interference. He sent for Achillas to gather 20,000 troops to fight for him.
Much to the shock and fury of Ptolemy and Pothinus, Cleopatra seduced Caesar and turned him to her side, and Ptolemy was soon taken captive. Open conflict broke out between the two sides, as Achillas marched on Alexandria and besieged Caesar and his 4,000 men inside the royal palace. Caesar sent emissaries to negotiate with Achillas, but knowing that Ptolemy did not consent to the negotiation, the enraged general executed them. He positioned his troops around the city, and skirmishes broke out.
As Caesar and his men held back Achillas' army, he decided to capture the Roman fleet in the port. Caesar was able to recapture the port, but was forced to burn his fleet, as he could not hold it. Caesar and his men pushed towards the Lighthouse of Alexandria, where Cleopatra's agent Aya lit the beacon, signalling Caesar's allies. Cleopatra's sister, Arsinoë, eventually joined Achillas, but near the end of the siege, she had him put to death by Ganymedes, a eunuch in service to Ptolemy.