A picture of SR388 from Metroid Fusion.

SR388 (sometimes SR-388, rarely SR 388) is a fictitious planet in the Metroid video game series. It is the Metroid creatures' home world and it is here where the second game of the series, Metroid II: Return of Samus, takes place. The current last game in the timeline, Metroid Fusion, takes place in a research station in orbit around this planet.

SR388's surface is rough, dry, and rocky. This world has many dangerous creatures and killer robots. Ancient ruins can be seen in the SR388 Underworld, which is what the lower levels of the planet are called. The ruins were built by Chozo and destroyed by Metroids.

At the time of Metroid II, the planet is overrun with all sorts of indigenous species as well as the Metroids. Samus' mission was to wipe out every last Metroid by exploring this vast, cavernous world. It may be hard to imagine any species being able to survive very long on the Metroids' home world – it is almost completely composed of rock, and the depths of the planet are full of deadly lava.

Before the Chozo came to SR388, the very efficient X Parasites were set to overrun the planet with their numbers. To keep them in check, the Chozo created the Metroid species and let them loose on the planet. The Metroids did keep the X in check very well, but when Samus Aran eradicated the species in Metroid II, the X came back in large numbers. One thing should be noted, as it may be a minor continuity issue: the X Parasites are first mentioned in Metroid Fusion, but they are never seen during the course of the Metroid II game.

At the end of Metroid Fusion, Samus set the Biologic Space Labs (B.S.L.) station to impact with SR388, which caused an explosion that vaporized both the planet and the station.