The new content contains more than two hours of additional gameplay, allowing fans to play as both Batman and Robin.
It is available now for 800 Microsoft Points (£6.80 / $10), before releasing on PSN and PC later this week, costing an estimated £7.99 / $9.99.
Taking place after the events of the main campaign, the DLC tasks players with stopping Harley Quinn's plans and shutting down the urban prison forever. It will feature new locations and enemies.
It precedes this year's Batman: Arkham City - Game of the Year Edition, which will be released in Europe on September 7.
Batman: Arkham City shipped more than 4.6 million copies during its first week on sale, and won the award for 'Best Action' game at this year's BAFTAs.
> Feature: 'Batman: Arkham City' - How the Dark Knight went open-world
> Read our review of 'Batman: Arkham City'
Photo gallery - Batman: Arkham City screens: