Champions Online is a free-to-play superhero-themed massively multiplayer online role-playing game (MMORPG) developed by Cryptic Studios for Microsoft Windows. The game is based on the Champions license and its ruleset is loosely based on the HERO System.

The game is an action-oriented role-playing game inspired by popular massively multiplayer games. Successful attacks and blocks increase a power meter which can then be spent to unleash more powerful actions. Death and defeat is handled in a similar manner to City of Heroes, in which a player may choose to be revived by a teammate or respawn at a neutral site. Players may sometimes gain the services of computer-controlled sidekicks and NPCs.

The game does not make use of functional armor or weapons, unlike most MMOs. Instead, clothing is chosen by the player, and damage and other stats are based on the character design, advancement, and gear. While some gear items may grant temporary powers or equipment or have a special visual effect, most simply provide stat enhancements without a visual representation. Reflecting the game's original design as a console game, characters also possess much fewer skills than other early MMOs; however, each skill can be customized with a variety of player-chosen "Advantages." The game features an "orb system" where enemies defeated by the player may drop orbs that temporarily benefit the hero's powers, life, or energy. Additionally, it includes basic non-combat crafting skills, based on Science, Arms, and Magic. Characters are granted at least one travel power, including a choice of swinging using grappling hooks, flying, rocket boot jumping, hover disks, super jump, super speed, teleportation, and acrobatics (fast running, leaps, and flips).

Various power and cosmetic choices can be unlocked, either through gameplay or through lockboxes. Lockboxes are activated by keys that can be bought with game currency using real world money.

Endgame content is available when the character reaches the level cap of 40.

The action takes place in the Champions universe, featuring Champions characters as NPCs. Regions include urban sprawls, a dinosaur-infested island, underground ruins, and underwater lairs. Each region in the game is composed of several "instances" with up to 35 players per instance at any one time. This allows the game to run on one huge virtual server and for players to travel freely between instances to join with other players, rather than the game being fragmented into different servers. Many missions take place within smaller instances created especially for a team entering them. These instances are reset if the player leaves and returns. It is possible to revisit the same region to redo some missions again and again.

A special region called "the underground circuit" allows player vs. player competition. Stronghold, the setting's advanced prison for super-powered inmates, is used for player vs. player scenarios in which the player will take on the role of a supervillain trying to break out or a superhero trying to contain them.

The game features a character creation system similar to City of Heroes, with additions such as the ability to edit a hero's movement animations, such as a beast-like hero opting to run and leap on all fours. The player may also edit the colour and firing position of their powers, such as changing a power's energy animation from red to green or changing the animation to fire from the head instead of the palms.