The drama is based on the 2002 book by Mara Leveritt and tells the story of three teenage kids, later known as 'The West Memphis Three' who were tried and convicted of killing three 8-year-old boys during an alleged satanic ritual.
The trio spent 18 years in jail, and one was sentenced to death, but all were subsequently freed in 2011.
The film, which stars another Academy Award winner Colin Firth, 51, "is about a crime that occurred in 1993," Witherspoon, 36, told The Chicago Sun Times. "It happened in Arkansas, and it's what I'd say is a very American story.
"After I finish Devil's Knot, I'm going to take a little time off" and go have a baby, Reese continued.