Metal Gear Online, commonly abbreviated MGO and also known as MGO2, was an online multiplayer spinoff of the Metal Gear series.

The game was officially unveiled in Kojima Productions' 20th Anniversary Party of the Metal Gear series. While Metal Gear Solid 3: Subsistence and Metal Gear Solid: Portable Ops featured optional online game modes that were both dubbed "Metal Gear Online", this incarnation of the online game was initially announced as a separate standalone product based on the Metal Gear Solid 4: Guns of the Patriots gameplay and setting. However, it was later announced as being a part of Metal Gear Solid 4. A standalone version was released in Japan on July 17, 2008.

On June 12, 2012, the servers for Metal Gear Online 2 were shut down, although fan servers have been created.