Achillas (Greek: Ἀχιλλᾶς; died 47 BC) was one of the guardians of the Egyptian king Ptolemy XIII Theos Philopator, and commander of the king's troops, when Pompey fled to Egypt in September 48 BC.

He was called by Julius Caesar a man of extraordinary daring, and it was he and Lucius Septimius who killed Pompey at the suggestion of the eunuch Pothinus and Theodotus of Chios.

[4] Caesar, who was at Alexandria, did not have sufficient forces to oppose him, and sent ambassadors to negotiate with him.

Meanwhile, Arsinoe, the younger sister of Ptolemy, escaped from Caesar and joined Achillas.

However, in 47 BC, dissension broke out between them, so Arsinoe had Achillas put to death by Ganymedes, a eunuch to whom she then entrusted the command of the forces.