The Alexandrian war was a phase of Caesar's civil war in which Julius Caesar involved himself in an Egyptian dynastic struggle. Caesar attempted to mediate a succession dispute between Cleopatra and Ptolemy XIII and exact repayment of certain Egyptian debts.

Arriving in Alexandria in October 48 BC and seeking initially to apprehend Pompey, his enemy in the civil war, Caesar found that Pompey had been assassinated by Ptolemy XIII's men. Caesar's financial demands and high-handedness then triggered a conflict which put him under siege in Alexandria's palace quarter. Only after external intervention from a Roman client state were Caesar's forces relieved. In the aftermath of Caesar's victory at the Battle of the Nile and Ptolemy XIII's death, Caesar installed his mistress Cleopatra as Egyptian queen, with her younger brother as co-monarch.