Assassin's Creed Origins is set mainly in Ptolemaic Egypt during the first century BCE.
The game follows Bayek of Siwa and his wife Aya as they investigate forces behind personal and political violence. Their journey takes place while Egypt is ruled by the Ptolemaic dynasty, a Greek royal house that governed the country after the death of Alexander the Great.
The setting includes Alexandria, Memphis, Cyrene, Giza, and the Siwa Oasis. Cleopatra VII and Julius Caesar appear as historical figures, placing the story in the final decades of the Ptolemaic kingdom and before Egypt became a Roman province.
Ptolemaic Egypt is sometimes loosely called ancient Egypt, but the more precise answer identifies the period and ruling dynasty. The game also presents a fictional origin story for the Hidden Ones, the organization that later develops into the Assassin Brotherhood.