Age of Empires Online is a multiplayer online real-time strategy game developed by Robot Entertainment and Gas Powered Games, which released on August 16, 2011. Based upon the gameplay of the Age of Empires series, it was originally developed by Robot Entertainment, but on February 24, 2011, Gas Powered Games, took over production. The game was published by Microsoft.

The game was free-to-play—it was free for users to play, though premium content could be earned or purchased, such as access to special items and blueprints, and more quests and features.

On March 27, 2012, the game premiered on Steam after a major overhaul patch which addressed complaints of an "MMO-like grind" and many other features, along with the debut of the Celtic civilization. On June 15, 2012 the premium content system was changed to offer the option to purchase all past and future content through "Empire points" earned in game, or by purchasing the points directly with real world money.

As of January 3, 2013, Microsoft ceased further development on the game. A Microsoft representative later announced on the forums that on August 22, 2013, the Games for Windows – Live Marketplace would be shut down, leaving the game only accessible by current or former players. In September 2013, it was announced that the game would remain functional until July 1, 2014 when the service would be shut down. The closure was due to the content being too expensive to maintain.

In 2017, Age of Empires Online was reestablished using Microsoft's publicly released developer-kit for the game and is now hosted on a server emulator known as "Project Celeste". The game can be played for free in single-player and multiplayer mode with all online features fully enabled. In May 2019, the team announced that they are developing Microsoft's unreleased Roman civilization for play based upon all known data of the civilization before the official servers were closed. It was finalized and released on March 14th, 2021