In The Matrix films, Zion is the last human city on Earth after a cataclysmic nuclear war between mankind and sentient machines, which resulted in artificial lifeforms dominating the world). It is a fictional city of over 250,000 humans, located deep under the ruined planets surface, close to the planets core, for warmth. The machines allowed Zion to be created as a method of coping with the problem of human choice, reducing but not eliminating the instability caused by those who rejected the Matrix. In order to prevent the entire system from crashing, the machines must periodically destroy Zion and "The One" must reboot the Matrix and choose a group of people to rebuild the city).

There are some fan theories that suggest that Zion is actually a second Matrix created by the machines to control the resistance and quarantine those who reject the Matrix proper. However, this theory has been debunked and rejected repeatedly.

In The Matrix Resurrections, Io is the new sister city of Zion. Niobe explains to Neo that in the 60 years following the end of the war, enough humans left the Matrix to cause a severe power shortage for the machines. The machines fought each other over the limited resources, but some of them sympathized with the displaced humans and helped them found the city of Io).