The single-player story expansion launched today on PC, Xbox 360 and PlayStation 3. The PlayStation 3 version will arrive with the regularly scheduled PSN store update, which means it will be available later today in North America, and on August 29 for Europe.
The Leviathan mission takes place during the course of the game's story, and sees Shepard pursuing rumours of a mysterious being said to be capable of defeating the Reapers.
Players will see new planets and star systems, new areas of the Citadel and meet new characters to further add to the game's story and flesh out the history of the Reapers.
One of the new environments for Leviathan will take Shepard underwater using a specialised mech suit.
The Leviathan DLC costs $9.99 (£6.80 / 800 MS Points) on PC, Xbox 360 and PlayStation 3 respectively.
Watch the trailer for Mass Effect 3's Leviathan DLC below: