Titled 'The Tyranny of King Washington', the DLC will continue the single-player story as three episodic content packs in an alternate history where George Washington seizes power as the King of America.
"History is our playground - and AC teams have always loved playing with historical facts and their consequences as a way to better understand a time period," Ubisoft executive producer Sebastien Puel said.
"While Assassin's Creed 3 concentrates on history as it happens, we wanted to take some liberties with this DLC and tell you how things 'could have happened'."
'The Tyranny of King Washington' will be included as part of the recently confirmed Assassin's Creed 3 season pass.
The season pass will sell separately from the game, granting access to five DLC packs including both single-player and multiplayer expansions. It will be priced at 2400 MS Points on Xbox 360, or £23.99 ($29.99) on PlayStation 3 and PC. All DLC will also be available individually for the Wii U version, though there will not be a Wii U season pass.
Officially unveiled in March, Assassin's Creed 3 will take place over a 30-year period in colonial America beginning in 1777.
It will be accompanied by PlayStation Vita spinoff Assassin's Creed 3: Liberation, which was announced at this year's E3 Gaming Expo.
Liberation will support cross-play with the PlayStation 3 edition of Assassin's Creed 3, allowing players to unlock bonuses in both games by trading goods.
Assassin's Creed 3 and Liberation will be released for Xbox 360, PlayStation 3 and PlayStation Vita on October 31 worldwide.
A Wii U version is also in development, and will be available during the system's launch window this holiday season.
The title is on course to break Ubisoft's record for the most pre-ordered game of all time.
> 'Assassin's Creed 3' preview: Three hours with the captivating sequel
> Digital Spy's 'Assassin's Creed 3' interview with Ubisoft
> Assassin's Creed 3 Liberation preview: Vita edition is dressed to kill
Watch the Assassin's Creed 3 Boston Tea Party trailer below: