Phantasy Star Online is an online role-playing game (RPG) developed by Sonic Team and published by Sega in 2000 for the Dreamcast. Players adventure with up to three others over the internet to complete quests, collect items and fight enemies in real-time action RPG combat. The story is unrelated to previous games in the Phantasy Star series.

Phantasy Star Online was the first successful online RPG for game consoles. Earlier online gaming was limited to western Windows games, particularly RPGs such as Diablo, Ultima Online and EverQuest. Believing online play was the future, Sega chairman Isao Okawa instructed Sonic Team to develop an online game for the Dreamcast, produced by Yuji Naka. Sonic Team's experiments led to the development of ChuChu Rocket!, the first online Dreamcast game. Using what they learned from the project, and taking significant inspiration from Diablo, Sonic Team built Phantasy Star Online. As Japanese internet service providers charged for dial-up access per minute, and high-speed connections were not yet widely available, Okawa personally paid for free internet access bundled with Japanese Dreamcasts.

Phantasy Star Online was highly anticipated and launched to positive reviews and commercial success; critics praised the online gameplay as addictive but criticized the single-player mode. It received the Japan Game Award for "Game of the Year" and is recognized as a landmark console game, influencing multiplayer dungeon crawlers, as well as the Monster Hunter series.

Phantasy Star Online was ported to Windows and rereleased on the Dreamcast as Ver. 2 with expanded content. Following Sega's exit from the console business in 2001, the game was ported to GameCube and Xbox as Episode I & II, featuring new characters, environments and other features. Episode III: C.A.R.D. Revolution, released for GameCube in 2003, was a turn-based card game. The online series continued with Phantasy Star Universe (2006) and Phantasy Star Online 2 (2012). Sega decommissioned the last official servers in 2010; Phantasy Star Online is still played on private servers.

Phantasy Star Online is an action role-playing game primarily played with other players cooperatively over the internet. Players take on the role of adventurers sent to explore Ragol, an uncharted planet. To create their character, they choose between a handful of races and classes, which define their abilities and statistics; for example, some types are better with "techniques" (magic spells) while others are more skilled with ranged or melee weapons.

Players can play either online or offline. Online, players are brought to a lobby where they can chat and organize teams of up to four. Team members can communicate by typing using a physical or onscreen keyboard, even when in different environments. Preset phrases are automatically translated between languages, and custom emoticons are also supported. Players can also exchange "guild cards" to exchange private messages and see when the other is online.

After organizing a team, players are transported to their own instance of the hub spaceship Pioneer 2, where they can buy and sell items, store money and items, heal, and browse quests. At any time, they can transport to Ragol, where combat and exploration take place. The four environments – forests, caves, mines, or ruins – comprise winding passages and large areas where enemies spawn. Players fight enemies in real time, using weapons and techniques and collecting items. Attacks can be chained for more accuracy and speed. Typically, all the monsters in a room must be defeated to advance. Each environment ends in a boss battle, which rewards the team with a large sum of experience points. Quests, taken on Pioneer 2, task players with specific challenges; once completed, the team returns to Pioneer 2 to collect their reward. Higher difficulties reward players with more experience points and better items. Some items can be used to feed the player's mag, a small creature that follows the player character and aids them in battle.

The story of Phantasy Star Online is unrelated to the original Phantasy Star series and is less substantial. Threatened by the imminent destruction of their home planet, thousands of refugees arrive at planet Ragol aboard the spaceship Pioneer 2. As they establish contact with colonists sent ahead on Pioneer 1, an enormous explosion shakes the planet. Adventurers from Pioneer 2 land to investigate the explosion and search for Rico Tyrell, daughter of the head of Pioneer 2. They discover the planet overrun by monsters, and follow messages left by Rico leading to an ancient evil, Dark Falz.