Final Fantasy XI Online is an MMORPG which is played in the persistent medieval world of Vana'diel. This base version was only released in Japan. The final North American release includes the Rise of the Zilart expansion pack and the XBox 360 version includes two more expansions: Chains of Promathia and Treasures of Aht Urhgan.

Create your own character by choosing your race, gender, face, body size and job. The five unique races are: Hume, Elvaan, Tarutaru, Mithra (female only), and Galka (male only).

Beginning jobs include: Warrior, Monk, Thief, White Mage, Black Mage and Red Mage. More jobs, such as Bard, Paladin, Beastmaster or Ranger, become available after certain quests are completed. Players can change their job at any time, but only in their Mog House, and each job has its own experience pool and level. They also each have unique traits that are acquired by leveling up, and their own set of abilities. The latter are not acquired by leveling, but by using scrolls bought from shops or acquired in special job quests. Each scroll requires a minimum level to be used. A second "support" job can also be set and changed at the Mog House by completing a special quest. The support job level is always maxed at the half of the main job level, rounded down. The character has access to all abilities, skills and traits from both jobs at their current level. The second job also adds to the characters stats. The maximum level a character can attain by gaining experience from battles and quest is 50, but, thanks to the game updates and add-ons, that maximum can eventually be raised to 55, 60, 65 and so forth, up to level 100 by completing the corresponding quests. At level 75, the character gains the ability to gather experience in a separate pool rather than the level pool, to earn merit points that can be spent to improve some aspects of the character.

Join other players to form a party of up to six players, on even an alliance of multiple parties, on various servers to fight the evil beastmen, the other species from Vana'diel (Orcs, Goblins, Quadavs, Yagudos, etc.). You can communicate with other players in your group using text and body language. A special device called a linkshell can be used by a group of players to communicate with each other, like in a chat channel.

Traveling through Vana'diel can be done by foot, on airships, riding a Chocobo, or by teleporting to the outposts scattered in each area. To teleport to a selected location, a supplies quest must first be completed. Next to each outpost is a book that enables special trainings. By defeating specific enemies, the character will gain extra experience and gils from those training manuals. Once engaged in a battle, the character will hit the targeted enemy if it is in range. Additional actions can be selected from a list, or used via a macro system. Aggressive foes will pursue their target until it has moved to another area.