Ultima is a series of open world fantasy role-playing video games from Origin Systems, created by Richard Garriott. Electronic Arts has owned the brand since 1992. The series had sold over 2 million copies by 1997.

Alongside Wizardry and Might and Magic, the Ultima series is considered to have established many norms of the computer role-playing game genre. Several games of the series are considered seminal in their genre. Their innovations, particularly in the early installments, were widely copied by other games.

The Ultima games are primarily within the scope of fantasy fiction but contain science fiction elements as well. They take place for the most part in a world called Britannia; the recurring hero is initially called the Stranger, until he attains the role of Avatar in Ultima IV and is known by that appellation from then on.

The main Ultima series consists of nine installments (with the seventh title divided into two parts) grouped into three trilogies, or "Ages": The Age of Darkness (Ultima I-III), The Age of Enlightenment (Ultima IV-VI), and The Age of Armageddon (Ultima VII-IX). The last is also sometimes referred to as "The Guardian Saga" after its chief antagonist. The first trilogy is set in a fantasy world named Sosaria, but during the cataclysmic events of The Age of Darkness, it is sundered and three-quarters of it vanish. What is left becomes known as Britannia, a realm ruled by the benevolent Lord British, and is where the later games mostly take place. The protagonist in all the games is a resident of Earth who is called upon by Lord British to protect Sosaria and, later, Britannia from a number of dangers. Originally, the player character was referred to as "the Stranger", but by the end of Ultima IV he becomes universally known as the Avatar.

In Ultima I: The First Age of Darkness (1981), the Stranger is first summoned to Sosaria to defeat the evil wizard Mondain who aims to enslave it. Since Mondain possesses the Gem of Immortality, which makes him invulnerable, the Stranger locates a time machine, travels back in time to kill Mondain before he creates the Gem, and shatters the incomplete artifact.

Ultima II: The Revenge of the Enchantress (1982) details Mondain's secret student and lover Minax's attempt to avenge him. When Minax launches an attack on the Stranger's homeworld of Earth, her actions cause doorways to open to various times and locations throughout Earth's history, and brings forth legions of monsters to all of them. The Stranger, after obtaining the Quicksword that alone can harm her, locates the evil sorceress at Castle Shadowguard at the origin of time and defeats her.

Ultima III: Exodus (1983) reveals that Mondain and Minax had an offspring, the eponymous Exodus, "neither human, nor machine", according to the later games (it is depicted as a computer at the conclusion of the game, and it appears to be a demonic, self-aware artificial intelligence). Some time after Minax's death, Exodus starts its own attack on Sosaria and the Stranger is summoned once again to destroy it. Exodus was the first installment of the series featuring a player party system, which was used in many later games.

Ultima IV: Quest of the Avatar (1985) marked a turning point in the series from the traditional "hero vs. villain" plots, instead introducing a complex alignment system based upon the Eight Virtues derived from the combinations of the Three Principles of Love, Truth and Courage. Although Britannia now prospers under Lord British's rule, he fears for his subjects' spiritual well-being and summons the Stranger again to become a spiritual leader of Britannian people by example. Throughout the game, the Stranger's actions determine how close he comes to this ideal. Upon achieving enlightenment in every Virtue, he can reach the Codex of Ultimate Wisdom and becomes the "Avatar", the embodiment of Britannia's virtues.