Originally scheduled for next week, the Mechromancer class will be available to download by the end of today (October 9) on Xbox 360, PS3 and PC.
The class is being offered as a free download to those who pre-ordered the game and joined the Borderlands 2 Premiere Club.
It can also be purchased separately for 800 Microsoft Points (£6.80 / $10).
The DLC will be followed by several other campaign add-ons planned between now and June 2013.
Released last month, Borderlands 2 was the biggest-selling launch of the year until the release of FIFA 13.
> Read Digital Spy's Borderlands 2 review
Watch the launch trailer for Borderlands 2 below: