The actress was told by legendary theatre producer Cameron Mackintosh at the end of a performance of Oliver! in Manchester last night (January 31) that she will play Eponine in Tom Hooper's star-studded movie.
The likes of Taylor Swift, Lea Michele, Scarlett Johansson and Evan Rachel Wood have all previously been linked to the role.
Barks wrote on Twitter that her casting represented the "most incredible moment of [her] life", adding this morning: "I have to say trying to sleep last night, I felt like a child on Christmas Eve!!!"
The 21-year-old previously played Eponine in the Les Misérables 25th anniversary concert at The O2 in 2010.
Barks rose to fame after finishing third in Andrew Lloyd Webber reality show I'd Do Anything in 2008. Jodie Prenger won the series and landed the role of Nancy in a West End revival of Oliver!.
Gossip Girl star Aaron Tevit and original Les Misérables cast members Colm Wilkinson and Frances Ruffelle recently joined the project, which already boasts Hugh Jackman, Russell Crowe, Helena Bonham Carter, Sacha Baron Cohen and Anne Hathaway among its stars.
Les Misérables is scheduled to begin shooting in March and will be released on December 7 in the US, with a UK release date to be confirmed.
Watch Samantha Barks perform 'On My Own' in the Les Misérables 25th anniversary concert below: