The actor's publicist has confirmed that Jackson will play a slave called Stephen in the movie, Variety reports.
The Pulp Fiction star will reunite with the film's director for the fifth time. Jackson has also appeared in Tarantino's Jackie Brown and Kill Bill Vol 2, in addition to an uncredited role as a narrator in Inglourious Basterds.
Django Unchained is a spaghetti western thought to be inspired by Sergio Corbucci's 1966 film Django.
Jamie Foxx is now confirmed in the lead role of freed slave Django after speculation that Will Smith or Idris Elba could secure the part. Leonardo DiCaprio will reportedly play Calvin Candie, a villainous plantation owner who holds Django's wife in his service.
Christoph Waltz - who won an Academy Award for his role in Inglourious Basterds - is also rumoured to be part of the cast.
Django Unchained is due to be released on Christmas Day 2012.
Watch the trailer for Inglourious Basterds below: