She will reprise her role from the first film in which she played Jade, a Las Vegas stripper who Stu (Ed Helms) married in the movie, reports Variety.
Graham did not appear in the second instalment of the franchise, which was set in Thailand. However, she will feature in the latest film, which brings the core cast back to the infamous Nevada city.
Stars Helms, Bradley Cooper, Zach Galifianakis and Justin Bartha will all be returning for the third and last film in the Hangover series.
Details of the plot remain tightly under wraps.
Graham recently starred in Judy Moody and the Not Bummer Summer. She will next be seen in About Cherry alongside Ashley Hinshaw and James Franco.
The Hangover: Part III is currently in pre-production, and will open in cinemas on May 24, 2013.
Watch Bradley Cooper and Ed Helms talking about The Hangover: Part II below: